Improve --timeout method to take into account all socket I/O that is

Improve --timeout method to take into account all socket I/O that is
going on.  The receiving side also switches timeout handling from the
receiver to the generator.  Given this setup, all keep-alive messages
are now sent as empty MSG_DATA messages, with MSG_NOOP messages only
being understood and (when necessary) acted upon to forward a keep-alive
event to an older receiver.  This is both safer and more compatible with
older versions.