Move channel-opening logic out into subroutines.

Each of the new subroutines corresponds to one of the channel types
for which we know how to parse a CHANNEL_OPEN, and has a collection of
parameters corresponding to the fields of that message structure.
ssh2_connection_filter_queue now confines itself to parsing the
message, calling one of those functions, and constructing an
appropriate reply message if any.

agentf.c

@@ -159,6 +159,7 @@ static const struct ChannelVtable agentf_channelvt = {
     chan_no_exit_status,
     chan_no_exit_signal,
     chan_no_exit_signal_numeric,
+    chan_no_request_response,
 };
 
 Channel *agentf_new(SshChannel *c)