List GWGs in Interface to send update from

Back-port of this fix done in master https://github.com/pfsense/pfsense/commit/31300a95f71b14dcb98c139388205223a36e8c8b and https://github.com/pfsense/pfsense/commit/8f56dd279432c4fd5a027310622e2650822e4651
Unfortunately this never got back-merged to 2.1 branch. A user on the forum noticed the issue again on 2.1.1 - https://forum.pfsense.org/index.php?topic=74922.0
I am submitting this so that when the next security issue happens, and there is 2.1.n+1 then this little fix will also happen automagically. I wonder what other fixes are in master that missed being back-ported to 2.1 branch.
Get real interface when dhcrelay uses default GW

If the DHCP Relay server is not on any local subnet, and not on any subnet that has an internal static route, but is somewhere that no specific route is known, then this code finds the default gateway and uses that in the DHCP relay "-i" parameter. The current code gets just the interface name (like "wan", "opt1"). But DHCP Relay command needs to be fed the actual device name "vr0", "vr1" etc.
To break this I simply enabled DHCP Relay, select "LAN" interface, and put Destination Server 1.2.3.4
Making this changed fixed it.
Forum: https://forum.pfsense.org/index.php?topic=75010.0
Fix typo
Merge pull request #1084 from phil-davis/patch-8
Merge pull request #1073 from phil-davis/patch-1
Merge pull request #1078 from phil-davis/patch-4