W_CheckProjectileDamage -- new global weapon function which check g_projectiles_damag...

W_CheckProjectileDamage -- new global weapon function which check g_projectiles_damage on whether or not a projectile can take a certain kind of damage.

// possible values:
// -2: absolutely no damage to projectiles (no exceptions)
// -1: no damage other than the exceptions (electro combo, hagar join explode, minelayer mines)
// 0: only damage from contents (lava/slime) or exceptions
// 1: only self damage or damage from contents or exceptions
// 2: allow all damage to projectiles normally