Two instances on one machine does not work

[shuckc]

Two instances currently clash on the ARTNET_PORT.
Try:

1. split to two listening sockets - at the moment we use one for both unicast and broadcast with wildcard address.
2. listen to the UDP broadcast IP/port with SO_REUSEADDR flag
3. pick a unique local port for the unicast communication (node to node) without SO_REUSEADDR.