New protocol: PROT_SSHCONN, bare ssh-connection.

This is the same protocol that PuTTY's connection sharing has been
using for years, to communicate between the downstream and upstream
PuTTYs. I'm now promoting it to be a first-class member of the
protocols list: if you have a server for it, you can select it in the
GUI or on the command line, and write out a saved session that
specifies it.

This would be completely insecure if you used it as an ordinary
network protocol, of course. Not only is it non-cryptographic and wide
open to eavesdropping and hijacking, but it's not even _authenticated_
- it begins after the userauth phase of SSH. So there isn't even the
mild security theatre of entering an easy-to-eavesdrop password, as
there is with, say, Telnet.

However, that's not what I want to use it for. My aim is to use it for
various specialist and niche purposes, all of which involve speaking
it over an 8-bit-clean data channel that is already set up, secured
and authenticated by other methods. There are lots of examples of such
channels:

 - a userv(1) invocation
 - the console of a UML kernel
 - the stdio channels into other kinds of container, such as Docker
 - the 'adb shell' channel (although it seems quite hard to run a
   custom binary at the far end of that)
 - a pair of pipes between PuTTY and a Cygwin helper process
 - and so on.

So this protocol is intended as a convenient way to get a client at
one end of any those to run a shell session at the other end. Unlike
other approaches, it will give you all the SSH-flavoured amenities
you're already used to, like forwarding your SSH agent into the
container, or forwarding selected network ports in or out of it, or
letting it open a window on your X server, or doing SCP/SFTP style
file transfer.

Of course another way to get all those amenities would be to run an
ordinary SSH server over the same channel - but this approach avoids
having to manage a phony password or authentication key, or taking up
your CPU time with pointless crypto.

ssh.c

@@ -311,8 +311,8 @@ static void ssh_got_ssh_version(struct ssh_version_receiver *rcv,
         ssh_connect_bpp(ssh);
 
         connection_layer = ssh2_connection_new(
-            ssh, NULL, false, ssh->conf, ssh_verstring_get_remote(old_bpp),
-            &ssh->cl);
+            ssh, ssh->connshare, false, ssh->conf,
+            ssh_verstring_get_remote(old_bpp), &ssh->cl);
         ssh_connect_ppl(ssh, connection_layer);
         ssh->base_layer = connection_layer;
     }
@@ -861,6 +861,11 @@ static void ssh_cache_conf_values(Ssh *ssh)
     ssh->pls.omit_data = conf_get_bool(ssh->conf, CONF_logomitdata);
 }
 
+bool ssh_is_bare(Ssh *ssh)
+{
+    return ssh->backend.vt->protocol == PROT_SSHCONN;
+}
+
 /*
  * Called to set up the connection.
  *
@@ -894,6 +899,8 @@ static const char *ssh_init(const BackendVtable *vt, Seat *seat,
     ssh->backend.vt = vt;
     *backend_handle = &ssh->backend;
 
+    ssh->bare_connection = (vt->protocol == PROT_SSHCONN);
+
     ssh->seat = seat;
     ssh->cl_dummy.logctx = ssh->logctx = logctx;
 
@@ -1194,3 +1201,25 @@ const struct BackendVtable ssh_backend = {
     PROT_SSH,
     22
 };
+
+const struct BackendVtable sshconn_backend = {
+    ssh_init,
+    ssh_free,
+    ssh_reconfig,
+    ssh_send,
+    ssh_sendbuffer,
+    ssh_size,
+    ssh_special,
+    ssh_get_specials,
+    ssh_connected,
+    ssh_return_exitcode,
+    ssh_sendok,
+    ssh_ldisc,
+    ssh_provide_ldisc,
+    ssh_unthrottle,
+    ssh_cfg_info,
+    ssh_test_for_upstream,
+    "ssh-connection", "Bare ssh-connection",
+    PROT_SSHCONN,
+    0
+};