examples/echo/{client,server}.scm: Update.

* examples/echo/server.scm (read-all): New procedure.
  (main): Update.  Catch exceptions.  Use `write-line' instead of
  `display'.
* examples/echo/client.scm (read-all): New procedure.
  (main): Update.  Use `char-ready?' instead of `channel-poll'.  Use
  `write-line' instead of `display'.