Add a startup switch to configure a multicast port

Proper cluster isolation with UDP requires use of a different cluster name, different multicast address and different multicast port. (See http://wiki.jboss.org/wiki/Wiki.jsp?page=PromiscuousTraffic for why the latter is important.)

The -g and -u switches allow easy configuration of the cluster name and multicast address. But, configuring the port that way wasn't practical, as an AS instance may have many different JChannels.

With the JGroups multiplexer, a switch is a possibility, as by default there will only be one JChannel per server instance. (If a user wanted to use more than one JChannel, they would need to manually configure the second one to use a different port.)