[cnc] MachineGun, APCgun air/ground. Weapons tweaked

Buffed MachineGun for Jeeps. Now identical to RA. 
(Default Humvee and Buggy are quite weak vs. infantry. This is meant to allow them to be useful in an anti-infantry role. They still remain balanced by being vulnerable to E3, tanks and vehicles)

APC gun can now shoot at ground and air.

Minor changes:
Airstrike and IonCannon tweaked to one-shot certain buildings (barracks & adv. power plant, respectively).
GrenadierExplode fixed.
Minor Housekeeping for Chemspray, some explosion sounds.