Make Only Missiles with MF2_FIRING Give Rings to Teammates | Allow MT_SPINFIRE to Deal Damage to Players Outside of Ringslinger (Resolves #164)

Solves both issues described in #164.

The fix for non-ring projectiles granting rings to teammates is done by giving MF2_FIRING new functionality when applied to objects with MF_MISSILE. P_FiringThink doesn't really work properly when used in combination with MF_MISSILE, so it was the perfect candidate for reuse in this scenario.

Before, all missiles without MF_FIRE would give rings, including Fang's cork.

Now, rings are only given if the missile has MF2_FIRING, which is automatically given to all of the vanilla Ringslinger projectiles when fired.

Plus, this behavior will now apply even in non-competitive gametypes, so you can give rings to your friends in any Co-op Ringslinger gametype.

In addition, MT_SPINFIRE will now deal damage to non-friendly players outside of Ringslinger, as described in the issue. This actually also applies to any object with MF_FIRE, such as fireballs, since the old behavior checked for that. Said old behavior seemed to be intentional, but it really doesn't make too much sense considering all other projectiles are allowed to deal damage.