examples/ssshd.scm, examples/sssh.scm: Update.

* examples/ssshd.scm (handle-channel): Use `read-all' instead of
  `channel-read'.  Update.
  (handle-request-exec): Use `display' instead of `channel-write'.
  (shell-loop): Use `read-all' instead of `channel-read' and
  `channel-poll'.   Use `display' instead of `channel-write'.
  (main): Close a channel before disconnecting the session.
* examples/sssh.scm (read-all): New procedure.
  (main): Use `read-all' instead of `read-line'.  Use `char-ready?'
  instead of `channel-poll'.