fixed M60A3 unable to climb gentle grades; MP engine sound issues

[NineLine]

The change that should fix this is set to be merged, but just missed this patch, some of the things related to the fix are related to changes in how CA controls vehicles. From the report:

Problem with units driving uphill 

• Clutch pedal input axis added
• Autoclutch implemented
• Automatic gearbox corrected
• gearbox parameters can be defined in unit script