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.

mainchan.c

@@ -37,6 +37,16 @@ static const struct ChannelVtable mainchan_channelvt = {
     mainchan_rcvd_exit_status,
     mainchan_rcvd_exit_signal,
     mainchan_rcvd_exit_signal_numeric,
+    chan_no_run_shell,
+    chan_no_run_command,
+    chan_no_run_subsystem,
+    chan_no_enable_x11_forwarding,
+    chan_no_enable_agent_forwarding,
+    chan_no_allocate_pty,
+    chan_no_set_env,
+    chan_no_send_break,
+    chan_no_send_signal,
+    chan_no_change_window_size,
     mainchan_request_response,
 };