Adds the match option to make missiles destructible

[luponix]

Adds the ability to shoot down missiles that are in flight.
Missiles have 50% of their total damage value as hitpoints currently based statically on zeros tested numbers.
There are two interactions:

Projectiles can shoot down Missiles.
The result depends on the damage and hp values.
-> if the damage of the projectile exceeds the remaining hp of the missile then the missile explodes and the projectile continues.
-> if the damage of the projectile is lower than the remaining hp of the missile then the projectile gets destroyed and the missile loses health equal to the projectiles damage.

Missiles alsocan shoot down Missiles:
In that case both missiles take damage. The damage amount equals the damage of the missile with the lower damage.
Missiles that fall below 0 hp in this collision explode.

Authority over the physics handling and subsequent result lies with the Server.
Only clients that send the corresponding mptweak receive information about the destruction of projectiles/missiles.

The option to enable this setting is in the match settings which we should totally expand to have multiple pages at some point :D if only that didnt mean having to unify all the spread out patches for that exact menu.

If this is well received this might make for a good addition for anarchy presets as it turned out to be less obstructive than expected and turning into a nice quality of life feature of being able to shoot down creepers or if you really try hard also some of the faster missiles