src/channel-type.c (ptob_input_waiting): Implement the procedure.

* src/channel-type.c (ptob_input_waiting): Call `ssh_channel_poll'.
  (init_channel_type): Register callback with
  `scm_set_port_input_waiting' procedure.
* examples/echo/server.scm (main): Use `char-ready?' predicate
  instead of the custom procedure `channel-poll'.