Move most of ssh.c out into separate source files.

I've tried to separate out as many individually coherent changes from
this work as I could into their own commits, but here's where I run
out and have to commit the rest of this major refactoring as a
big-bang change.

Most of ssh.c is now no longer in ssh.c: all five of the main
coroutines that handle layers of the SSH-1 and SSH-2 protocols now
each have their own source file to live in, and a lot of the
supporting functions have moved into the appropriate one of those too.

The new abstraction is a vtable called 'PacketProtocolLayer', which
has an input and output packet queue. Each layer's main coroutine is
invoked from the method ssh_ppl_process_queue(), which is usually
(though not exclusively) triggered automatically when things are
pushed on the input queue. In SSH-2, the base layer is the transport
protocol, and it contains a pair of subsidiary queues by which it
passes some of its packets to the higher SSH-2 layers - first userauth
and then connection, which are peers at the same level, with the
former abdicating in favour of the latter at the appropriate moment.
SSH-1 is simpler: the whole login phase of the protocol (crypto setup
and authentication) is all in one module, and since SSH-1 has no
repeat key exchange, that setup layer abdicates in favour of the
connection phase when it's done.

ssh.c itself is now about a tenth of its old size (which all by itself
is cause for celebration!). Its main job is to set up all the layers,
hook them up to each other and to the BPP, and to funnel data back and
forth between that collection of modules and external things such as
the network and the terminal. Once it's set up a collection of packet
protocol layers, it communicates with them partly by calling methods
of the base layer (and if that's ssh2transport then it will delegate
some functionality to the corresponding methods of its higher layer),
and partly by talking directly to the connection layer no matter where
it is in the stack by means of the separate ConnectionLayer vtable
which I introduced in commit 8001dd4cb, and to which I've now added
quite a few extra methods replacing services that used to be internal
function calls within ssh.c.

(One effect of this is that the SSH-1 and SSH-2 channel storage is now
no longer shared - there are distinct struct types ssh1_channel and
ssh2_channel. That means a bit more code duplication, but on the plus
side, a lot fewer confusing conditionals in the middle of half-shared
functions, and less risk of a piece of SSH-1 escaping into SSH-2 or
vice versa, which I remember has happened at least once in the past.)

The bulk of this commit introduces the five new source files, their
common header sshppl.h and some shared supporting routines in
sshcommon.c, and rewrites nearly all of ssh.c itself. But it also
includes a couple of other changes that I couldn't separate easily
enough:

Firstly, there's a new handling for socket EOF, in which ssh.c sets an
'input_eof' flag in the BPP, and that responds by checking a flag that
tells it whether to report the EOF as an error or not. (This is the
main reason for those new BPP_READ / BPP_WAITFOR macros - they can
check the EOF flag every time the coroutine is resumed.)

Secondly, the error reporting itself is changed around again. I'd
expected to put some data fields in the public PacketProtocolLayer
structure that it could set to report errors in the same way as the
BPPs have been doing, but in the end, I decided propagating all those
data fields around was a pain and that even the BPPs shouldn't have
been doing it that way. So I've reverted to a system where everything
calls back to functions in ssh.c itself to report any connection-
ending condition. But there's a new family of those functions,
categorising the possible such conditions by semantics, and each one
has a different set of detailed effects (e.g. how rudely to close the
network connection, what exit status should be passed back to the
whole application, whether to send a disconnect message and/or display
a GUI error box).

I don't expect this to be immediately perfect: of course, the code has
been through a big upheaval, new bugs are expected, and I haven't been
able to do a full job of testing (e.g. I haven't tested every auth or
kex method). But I've checked that it _basically_ works - both SSH
protocols, all the different kinds of forwarding channel, more than
one auth method, Windows and Linux, connection sharing - and I think
it's now at the point where the easiest way to find further bugs is to
let it out into the wild and see what users can spot.

ssh.h

@@ -183,7 +183,18 @@ void share_setup_x11_channel(ssh_sharing_connstate *cs, share_channel *chan,
                              int protomajor, int protominor,
                              const void *initial_data, int initial_len);
 
-struct ssh_rportfwd;
+/* Structure definition centralised here because the SSH-1 and SSH-2
+ * connection layers both use it. But the client module (portfwd.c)
+ * should not try to look inside here. */
+struct ssh_rportfwd {
+    unsigned sport, dport;
+    char *shost, *dhost;
+    int addressfamily;
+    char *log_description; /* name of remote listening port, for logging */
+    ssh_sharing_connstate *share_ctx;
+    PortFwdRecord *pfr;
+};
+void free_rportfwd(struct ssh_rportfwd *rpf);
 
 struct ConnectionLayerVtable {
     /* Allocate and free remote-to-local port forwardings, called by
@@ -227,6 +238,25 @@ struct ConnectionLayerVtable {
 
     /* Query whether the connection layer is doing agent forwarding */
     int (*agent_forwarding_permitted)(ConnectionLayer *cl);
+
+    /* Set the size of the main terminal window (if any) */
+    void (*terminal_size)(ConnectionLayer *cl, int width, int height);
+
+    /* Indicate that the backlog on standard output has cleared */
+    void (*stdout_unthrottle)(ConnectionLayer *cl, int bufsize);
+
+    /* Query the size of the backlog on standard _input_ */
+    int (*stdin_backlog)(ConnectionLayer *cl);
+
+    /* Tell the connection layer that the SSH connection itself has
+     * backed up, so it should tell all currently open channels to
+     * cease reading from their local input sources if they can. (Or
+     * tell it that that state of affairs has gone away again.) */
+    void (*throttle_all_channels)(ConnectionLayer *cl, int throttled);
+
+    /* Ask the connection layer about its current preference for
+     * line-discipline options. */
+    int (*ldisc_option)(ConnectionLayer *cl, int option);
 };
 
 struct ConnectionLayer {
@@ -253,6 +283,13 @@ struct ConnectionLayer {
     ((cl)->vt->sharing_queue_global_request(cl, cs))
 #define ssh_agent_forwarding_permitted(cl) \
     ((cl)->vt->agent_forwarding_permitted(cl))
+#define ssh_terminal_size(cl, w, h) ((cl)->vt->terminal_size(cl, w, h))
+#define ssh_stdout_unthrottle(cl, bufsize) \
+    ((cl)->vt->stdout_unthrottle(cl, bufsize))
+#define ssh_stdin_backlog(cl) ((cl)->vt->stdin_backlog(cl))
+#define ssh_throttle_all_channels(cl, throttled) \
+    ((cl)->vt->throttle_all_channels(cl, throttled))
+#define ssh_ldisc_option(cl, option) ((cl)->vt->ldisc_option(cl, option))
 
 /* Exports from portfwd.c */
 PortFwdManager *portfwdmgr_new(ConnectionLayer *cl);
@@ -266,6 +303,19 @@ char *portfwdmgr_connect(PortFwdManager *mgr, Channel **chan_ret,
 
 Frontend *ssh_get_frontend(Ssh ssh);
 
+/* Communications back to ssh.c from connection layers */
+void ssh_throttle_conn(Ssh ssh, int adjust);
+void ssh_got_exitcode(Ssh ssh, int status);
+void ssh_ldisc_update(Ssh ssh);
+void ssh_got_fallback_cmd(Ssh ssh);
+
+/* Functions to abort the connection, for various reasons. */
+void ssh_remote_error(Ssh ssh, const char *fmt, ...);
+void ssh_remote_eof(Ssh ssh, const char *fmt, ...);
+void ssh_proto_error(Ssh ssh, const char *fmt, ...);
+void ssh_sw_abort(Ssh ssh, const char *fmt, ...);
+void ssh_user_close(Ssh ssh, const char *fmt, ...);
+
 #define SSH_CIPHER_IDEA		1
 #define SSH_CIPHER_DES		2
 #define SSH_CIPHER_3DES		3
@@ -1241,10 +1291,6 @@ enum {
 #undef DEF_ENUM_UNIVERSAL
 #undef DEF_ENUM_CONTEXTUAL
 
-/* Given that virtual packet types exist, this is how big the dispatch
- * table has to be */
-#define SSH_MAX_MSG                           0x101
-
 /*
  * SSH-1 agent messages.
  */