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

* examples/echo/client.scm: Update comments.
* examples/echo/server.scm: Likewise.
channel-type.c (ptob_input_waiting, ptob_fill_input): Improve.

* src/channel-type.c (ptob_input_waiting): Improve error checking.
  (ptob_fill_input): Call `ssh_channel_poll' to update underlying
  SSH channel state and check whether we have data to read or not.
  Check if the channel is open.
TODO: Update.