Gracefully handle reads from channels with data not yet available.

* ssh/channel-type.c (ptob_fill_input): Check
  `ssh_channel_is_open' first.  Consider only SSH_ERROR and
  SSH_EOF as special return values of `scm_channel_poll'; a return
  value of zero is OK.  Likewise, don't return EOF when
  `ssh_channel_read' returns zero.
Merge branch 'master' into ludo-gracefully-handle-reads-from-channels

Conflicts:
	ChangeLog
examples/{sssh,ssshd}.scm.in: Improve error handling.

* examples/sssh.scm.in (read-all): Improve error handling: Catch
  `guile-ssh-error'.
* examples/ssshd.scm.in (read-all): Likewise.
* NEWS: Update.
Thank Ludovic.

* THANKS: Add to the repository.  Thank Ludovic.
* AUTHORS: Update.