Fixes https://github.com/Khopa/dcs_liberation/issues/473. Air defenses
for bases, strike locations, and fixed IADS will now all downgrade to
lower tier systems as needed. Strike locations will still be spawned as
an equally weighted random generator from either the medium or long
range groups, but will use a short range system if none are available to
the faction.
I've made the change in a way that leaves factions compatible, but will
follow up to clean up our built-in factions.
Enabled for bluefor modern, since they ought to have GPS but seemingly
don't.
This change does nothing until https://github.com/pydcs/dcs/pull/102
lands and we update to it.
