ssh/channel-type.h (GSSH_VALIDATE_OPEN_CHANNEL): New macro

* ssh/channel-type.h (GSSH_VALIDATE_OPEN_CHANNEL): New macro.
* ssh/channel-func.c (guile_ssh_channel_request_exec)
  (guile_ssh_channel_request_pty, guile_ssh_channel_request_shell)
  (guile_ssh_channel_request_env, guile_ssh_channel_set_pty_size_x)
  (guile_ssh_channel_set_stream_x, guile_ssh_channel_get_stream): Expect
  an open channel as an argument.
* doc/api-channels.texi (Channels): Update.