Avoid useless keepalive msgs that would kill an older rsync.

Avoid useless keepalive msgs that would kill an older rsync.

This fix avoids the sending of keep-alive messages from the receiver
to the sender when we are still sending the file list (at which time
an older rsync would die if it received such a keep-alive message).
The messages aren't actually needed, since we haven't forked yet, and
the single flow of data keeps the procs alive.