Make UDP packet reception work right with non-blocking sockets and enable.

We need this to permit a UDP receiving iperf3 server to listen on its
control channel.

The fix for non-blocking sockets basically makes sure that if we do a
read on a non-blocking sockets, but there's no data, the UDP processing
code does *not* try to do the normal protocol packet processing on the
non-existent packet.

This is part of an ongoing fix for issue #212 but also should have been
a part of the fix for issue #125.