Server prep: parse a lot of new channel requests.

ssh2connection.c now knows how to unmarshal the message formats for
all the channel requests we'll need to handle when we're the server
and a client sends them. Each one is translated into a call to a new
method in the Channel vtable, which is implemented by a trivial
'always fail' routine in every channel type we know about so far.

ssh1connection-client.c

@@ -429,6 +429,9 @@ static const struct SshChannelVtable ssh1mainchan_vtable = {
     NULL /* get_conf */,
     NULL /* window_override_removed is only used by SSH-2 sharing */,
     NULL /* x11_sharing_handover, likewise */,
+    NULL /* send_exit_status */,
+    NULL /* send_exit_signal */,
+    NULL /* send_exit_signal_numeric */,
     ssh1mainchan_request_x11_forwarding,
     ssh1mainchan_request_agent_forwarding,
     ssh1mainchan_request_pty,