number of remote args.
- We now exchange a "VERSION.SUBVERSION" value in the initial @RSYNCD
  message.  This is backward compatible with pre-protocol 30 rsyncs,
  and fixes a problem where rsync daemons weren't checking if they were
  speaking a compatible pre-release protocol.
- Moved the protocol-handling code (which was in two places) into a new
  function:  exchange_protocols().
- Protocol 30 now sends/receives the daemon args using NULLs as a separator
  rather than a newline.
- Moved the arg-reading code into a new function in io.c:  read_args().