message-func.c (guile_ssh_message_get_req): Handle SSH_REQUEST_CHANNEL_OPEN.

* src/message.scm (channel-open-req:orig, channel-open-req:orig-port)
  (channel-open-req:dest, channel-open-req:dest-port): New procedures
* src/message-func.c (guile_ssh_message_get_req): Handle
  SSH_REQUEST_CHANNEL_OPEN.
  (get_channel_open_req): New procedure.