rsync spawner: Poll all read file descriptors

rsync spawner: Poll all read file descriptors

The single thread requirement and lack of polling was preventing the
spawner from running multiple rsyncs at the same time (as their output
needs to be exhausted for them to end), and more importantly, from
consuming the request stream while the one rsync was running. (The
latter might result in dropped requests if too many rsyncs are queued.)

Therefore, poll both the request stream and the rsync pipes. All input
is now consumed immediately, and multiple rsyncs can be forked at the
same time. (Still needs a limit.)