The mailbox module currently does nothing to ensure
messages/indexes are physically on disk when the
flush() method returns or message files are closed. 
This patch adds functions _sync_flush and _sync_close
to flush and fsync() a file object, and in the latter
case close it afterwards.  _sync_close is then used
where needed throughout the code.  (For various reasons
the current implementation only ever requires a sync
immediately before closing a file, but _sync_flush is
provided for future use.)