Enabled SO_REUSEADDR socket option of the ArtNetServer listener

This allows two applications, or more, to bind to the same socket, thus
listening and reacting to the same DMX Input packets. It only works as
long as all of the application also set the SO_REUSEADDR socket option.

See also:
http://download.java.net/jdk7/archive/b123/docs/api/java/net/DatagramSoc
ket.html#setReuseAddress(boolean)