MobjDamage hook tweaks for special stages

Resolves #822 (closed) .

This MR makes the MobjDamage lua hook actually works for non-NiGHTS special stages, also supports return true.

It also tweaks the hook's function for both old-style specials and NiGHTS to avoid it being called when flashing.

Test files:

mobjdamage_test1.wad mobjdamage_test2.wad

About the test files:

Each time the player gets hurt, it outputs "playerdamage" to the console.

'test2' is just 'test1' with return true inside the hook.