ssh/: Improve printing of Guile-SSH objects

* ssh/channel-type.c (print_channel): Print data type first.  Print
  `freed' state if the channel has been closed and freed.
* ssh/key-type.c (print_key): Print data type first.
* ssh/message-type.c (print_message): Improve.
* ssh/session-type.c (print_session): Improve a bit.
TODO: Update