Refactor the LogContext type.

LogContext is now the owner of the logevent() function that back ends
and so forth are constantly calling. Previously, logevent was owned by
the Frontend, which would store the message into its list for the GUI
Event Log dialog (or print it to standard error, or whatever) and then
pass it _back_ to LogContext to write to the currently open log file.
Now it's the other way round: LogContext gets the message from the
back end first, writes it to its log file if it feels so inclined, and
communicates it back to the front end.

This means that lots of parts of the back end system no longer need to
have a pointer to a full-on Frontend; the only thing they needed it
for was logging, so now they just have a LogContext (which many of
them had to have anyway, e.g. for logging SSH packets or session
traffic).

LogContext itself also doesn't get a full Frontend pointer any more:
it now talks back to the front end via a little vtable of its own
called LogPolicy, which contains the method that passes Event Log
entries through, the old askappend() function that decides whether to
truncate a pre-existing log file, and an emergency function for
printing an especially prominent message if the log file can't be
created. One minor nice effect of this is that console and GUI apps
can implement that last function subtly differently, so that Unix
console apps can write it with a plain \n instead of the \r\n
(harmless but inelegant) that the old centralised implementation
generated.

One other consequence of this is that the LogContext has to be
provided to backend_init() so that it's available to backends from the
instant of creation, rather than being provided via a separate API
call a couple of function calls later, because backends have typically
started doing things that need logging (like making network
connections) before the call to backend_provide_logctx. Fortunately,
there's no case in the whole code base where we don't already have
logctx by the time we make a backend (so I don't actually remember why
I ever delayed providing one). So that shortens the backend API by one
function, which is always nice.

While I'm tidying up, I've also moved the printf-style logeventf() and
the handy logevent_and_free() into logging.c, instead of having copies
of them scattered around other places. This has also let me remove
some stub functions from a couple of outlying applications like
Pageant. Finally, I've removed the pointless "_tag" at the end of
LogContext's official struct name.

sshbpp.h

@@ -23,7 +23,6 @@ struct BinaryPacketProtocol {
     PacketLogSettings *pls;
     LogContext *logctx;
     Ssh *ssh;
-    Frontend *frontend;
 
     /* ic_in_raw is filled in by the BPP (probably by calling
      * ssh_bpp_common_setup). The BPP's owner triggers it when data is
@@ -53,7 +52,7 @@ struct BinaryPacketProtocol {
  * does centralised parts of the freeing too. */
 void ssh_bpp_free(BinaryPacketProtocol *bpp);
 
-BinaryPacketProtocol *ssh1_bpp_new(Frontend *frontend);
+BinaryPacketProtocol *ssh1_bpp_new(LogContext *logctx);
 void ssh1_bpp_new_cipher(BinaryPacketProtocol *bpp,
                          const struct ssh1_cipheralg *cipher,
                          const void *session_key);
@@ -72,6 +71,13 @@ void ssh2_bpp_queue_disconnect(BinaryPacketProtocol *bpp,
                                const char *msg, int category);
 int ssh2_bpp_check_unimplemented(BinaryPacketProtocol *bpp, PktIn *pktin);
 
+/* Convenience macro for BPPs to send formatted strings to the Event
+ * Log. Assumes a function parameter called 'bpp' is in scope, and
+ * takes a double pair of parens because it passes a whole argument
+ * list to dupprintf. */
+#define bpp_logevent(params) ( \
+        logevent_and_free((bpp)->logctx, dupprintf params))
+
 /*
  * Structure that tracks how much data is sent and received, for
  * purposes of triggering an SSH-2 rekey when either one gets over a
@@ -98,7 +104,7 @@ struct DataTransferStats {
      ((stats)->direction.remaining -= (size), FALSE))
 
 BinaryPacketProtocol *ssh2_bpp_new(
-    Frontend *frontend, struct DataTransferStats *stats);
+    LogContext *logctx, struct DataTransferStats *stats);
 void ssh2_bpp_new_outgoing_crypto(
     BinaryPacketProtocol *bpp,
     const struct ssh2_cipheralg *cipher, const void *ckey, const void *iv,
@@ -121,7 +127,7 @@ void ssh2_bpp_new_incoming_crypto(
  */
 int ssh2_bpp_rekey_inadvisable(BinaryPacketProtocol *bpp);
 
-BinaryPacketProtocol *ssh2_bare_bpp_new(Frontend *frontend);
+BinaryPacketProtocol *ssh2_bare_bpp_new(LogContext *logctx);
 
 /*
  * The initial code to handle the SSH version exchange is also
@@ -134,7 +140,7 @@ struct ssh_version_receiver {
                             int major_version);
 };
 BinaryPacketProtocol *ssh_verstring_new(
-    Conf *conf, Frontend *frontend, int bare_connection_mode,
+    Conf *conf, LogContext *logctx, int bare_connection_mode,
     const char *protoversion, struct ssh_version_receiver *rcv);
 const char *ssh_verstring_get_remote(BinaryPacketProtocol *);
 const char *ssh_verstring_get_local(BinaryPacketProtocol *);