Uppity: free the packet protocol layers on exit.

This bug and the one in the previous commit combined to mean that when
an SSH-1 connection through Uppity is terminated, the pty backend for
its main session channel was never cleaned up. (Firstly because
ssh1_connection_free never got called, and secondly because that in
turn forgot to free its mainchan.)

The effect of that in turn was that a _subsequent_ connection to the
same Uppity (using the new listening-socket mode) would likely reuse
the same fd for its pty, and the insertions into the ptyfds tree in
uxpty.c would silently fail because an existing Pty was already
occupying them, leading to a segfault when that Pty in turn responded
to events on a pty it didn't really own and tried to call back to a
seat that didn't exist any more.

sshserver.c

@@ -311,6 +311,8 @@ static void ssh_server_free_callback(void *vsrv)
 
     sk_close(srv->socket);
 
+    if (srv->base_layer)
+        ssh_ppl_free(srv->base_layer);
     if (srv->bpp)
         ssh_bpp_free(srv->bpp);