Enable use of DNS-SD to find tracker Internet socket address

[semmerson]

Currently, the Internet socket address of a tracker is assumed to use port number 3800 unless otherwise specified in the XML subscription. DNS-SD can be used to eliminate this dependency on an assigned port number. Subscriber should be modified to first try DNS-SD for the Sruth service in the domain specified in the XML subscription and, if that fails, try port 38800.