Partial damage system overhaul: P_DamageMobj, P_KillMobj, P_HitDeathMessages...

Partial damage system overhaul: P_DamageMobj, P_KillMobj, P_HitDeathMessages and P_RingDamage now all take a "damagetype" variable that determines what type of damage was dealt. Does not replace the "damage" variable, we'll have to see if that is worth keeping another time?

Currently the main benefit of these changes is that a number of non-Object-related hazards/deaths no long rely on dummy MT_NULL objects or other hacks to tell the game how you were hurt/killed, yay

git-svn-id: https://code.orospakr.ca/svn/srb2/trunk@9039 6de4a73c-47e2-0310-b8c1-93d6ecd3f8cd