Get UDP working.

Get UDP working.

For UDP, activate retransmit timer before receiving rather than
sending makes the event loop break nicely after sending a message
(which is important for blocking mode).  Not quite sure that this is
really accurate wrt to retransmission timing though but it should do
for now.

For UDP, set the user_data member for the read callback in
rs_conn_receive_packet -- the one from udp_init() doesn't do much good
now.

For UDP, implement receiving message.

Add compat_recv().