Fix GC'ing of SSH objects.

The program doesn't crashes anymore during GC'ing of channels and
keys.

* src/session-type.c (free_session): Mark all related channels as
  freed.
  (guile_ssh_make_session): Initialize the channels array.
* src/key-type.c (mark_key_smob): Fix smob marking.
* src/channel-type.c (free_channel): Check if the channel has been
  already freed along with the related SSH session.
  (guile_ssh_make_channel): Store the reference to the channel in
  the array of channels related to the SSH session.
* src/channel-type.h: Add is_channel_alive field to channel_data.
* TODO: Add to the repository.