Adding a time-based self-destruct variable to missiles.

Note : All missiles that don't have a designated time are defaulted to self-destruct after 10 seconds.

Exceptions:

Grenadier RPG : 5 seconds (compensated by more building damage)

Nuclear Missile : 20 seconds (might change, this is to block airborne explosion)

Upgraded ML/Heavy-duty ML : 5 seconds.