Rewrite the SSH-1 main shell session using mainchan.

In SSH-1, the channel system isn't rich enough to represent the
complicated main shell session, so it's all done with a separate set
of custom message types. But PuTTY now abstracts away that difference,
by representing both as different implementations of the SshChannel
class: ssh1channel is for things that the protocol thinks are 'really'
channels, and ssh1mainchan is for the shell session. All the same
methods are implemented, but generate different wire messages.

This means that the logic to decide _when_ to enable X forwarding,
agent forwarding etc is all centralised into mainchan.c, where it
doesn't have to be repeated for both protocol versions.

It also simplifies the final loop in the connection protocol, which no
longer has to contain the code to move data from the user input
bufchain to the channel's output; that's now done by the mainchan
write method, the same as it is in SSH-2 where mainchan is just like
other channels.

mainchan.c

@@ -95,7 +95,7 @@ mainchan *mainchan_new(
         mc->type = MAINCHAN_SESSION;
     }
 
-    *sc_out = mc->sc;
+    if (sc_out) *sc_out = mc->sc;
     return mc;
 }