Problem: hydrad has to announce itself on LAN

Solution: use UDP beaconing (via CZMQ zbeacon class)

- broadcast UDP frame on port 5670 once per second
- frame contains 8 bytes: [H][Y][D][R][A][0x01][port] where the port
  is two-byte value in network order.

Recipient gets hydrad network address via UDP recvfrom, and uses port
to know where to make its TCP connection via ZeroMQ.

Note: UDP beaconing and TCP protocol should eventualy be written up
as an RFC along similar lines as ZRE RFC 36.