Return GWG IP protocol (version) when no gateway IP

This is a resubmit of an older pull request I had closed (883). It came up again in the forum, with a user setting up a VM environment and wondering why he cannot make policy-routing rules to a gateway group. The gateway group names did not display because all his WANs were down at the time and so the existing code was too dumb to determine the IP protocol (version 4 or 6) when none of the WANs in the gateway group actually have an IP address yet. This fixes that behavior. return_gateway_groups_array() can now return the GWG IP protocol and also have all member gateways down.
If you are happy with this, I will also make a version for 2.1 branch so it can be fixed there.
Forum references:
http://forum.pfsense.org/index.php/topic,71142.0.html
https://forum.pfsense.org/index.php/topic,73144.0.html