ssh/channel.scm: Improve forwarding procedures

* ssh/channel-func.c (channel-open-forward): Rename to
  `%channel-open-forward'.  Rename arguments.
  (channel-open-reverse-forward): Rename to
  `%channel-open-forward/reverse'.  Rename arguments.
* ssh/channel.scm (channel-open-forward): Rename to
  `%channel-open-forward'.
  (channel-open-reverse-forward): Rename to
  `%channel-open-forward/reverse'.
  (channel-open-forward, channel-open-forward/reverse): New procedures.