I've added a systemd service file for openSUSE >= 12.3.

Previous systemd based versions would require an additional variant of the service file, because SuSEfirewall2.service (which is an ordering dependency) used to be named SuSEfirewall2_setup.service in these versions. I don't think it would be worth the trouble of adding yet another switch, but I can of course do so if desired. (forwarded request 199913 from marc_schuetz)