fixed bug where obsolete service files werent being deleted when service...

... is updated

to test:
1) create a service w/ web interface, where service IP address is 127.0.0.1 and Port field is blank. You should see a file in /etc/avahi/services/ called 127_460_460_461.service.
2) Log in as admin, and edit the service in the web interface. Add '80' to the Port field, and click submit. In the /etc/avahi/services directory, the old file 127_460_460_461.service should no longer be there, and 127_460_460_46180.service should be there instead.