- The receiver now sends keep-alive messages to the generator
  when it is actively doing work and hasn't sent anything
  recently.  This ensures that the generator won't timeout
  if the receiver is working hard.
- The perform_io() code has improved keep-alive participation.
- Allow the sender to send some keep-alive messages, which
  ensures that if it is in a lull, it can probe the socket.

the git-status checking into a library.

so that 3.0.7 knows we support the safer flist-xfer method.

The receiving side also switches timeout handling from the receiver to
the generator, which obviates the need for the sender to send any
keep-alive messages at all (for protocol 31 and beyond).  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.

MSG_NOOP is not available (is both safer and supports older rsyncs).

and no one is reading (i.e. do advantageous reading in perform_io()).

Added some debug output for MSG_ERROR_EXIT messages.