Enabled SO_REUSEADDR socket option of the ArtNetServer listener

Setting this option allows two applications, or more, to bind to the same socket, thus to listen and react to the same DMX Input packets. It only works as long as all of the application also set the SO_REUSEADDR socket option.

I used it successfully to have MadMapper (a _projection mapping_ application written in C, which obviously sets the SO_REUSEADDR socket option) and a Processing application (written in Processing/Java and using an Artnet4j-Elios library patched with this change) reacting to the same DMX Input events, which were sent from a small lightning console via DMX and received as ArtNet datagrams via Enttec's OpenDMX Ethernet box on the computer running the apps.

See also:
http://download.java.net/jdk7/archive/b123/docs/api/java/net/DatagramSocket.html#setReuseAddress(boolean)