From Axem: add hud-set-custom-gauge-active and hud-set-retail-gauge active

Deprecates hud-set-active-gauge and hud-activate-gauge-type in favor of hud-set-custom-gauge-active and hud-set-retail-gauge active, which are clearer to understand and now accept INT_MAX number of gauges, instead of just 1.

git-svn-id: svn://svn.icculus.org/fs2open@10489 387891d4-d844-0410-90c0-e4c51a9137d3
Renamed "$Max Shield Recharge Percent" to "$Max Shield Recharge", along with the associated variables.

git-svn-id: svn://svn.icculus.org/fs2open@10490 387891d4-d844-0410-90c0-e4c51a9137d3