- Updated many functions.

- Added A_KillTarget(damagetype, int flags).
- Added A_KillTracer(damagetype, int flags).
- Added A_RemoveTarget.
- Added A_RemoveTracer.

A_Kill (Master/Target/Tracer/Children/Siblings):
- KILS_FOILINVUL: foils invulnerability.
- KILS_KILLMISSILES: destroys projectiles. Does not work on invulnerable
projectiles without KILS_FOILINVUL, and doesn't work at all on missiles
with NODAMAGE flag.
- KILS_NOMONSTERS: actors that are monsters will not be killed.

A_Damage (Self/Target/Master/Tracer/Children/Siblings):
- DMSS_FOILINVUL: foils invulnerability.
- DMSS_AFFECTARMOR: damages the actor's armor instead of bypassing it
entirely.
- DMSS_KILL: damages the actor by its remaining health (useful for
modular DECORATE programming).

- Added A_SpawnItemEx flags:
- SXF_SETTARGET: sets the calling actor as the target.
- SXF_SETTRACER: sets the calling actor as the tracer.
Both of these functions take priority similar to SXF_SETMASTER over
SXF_TRANSFERPOINTERS.