Add PickActor ACS function.

PickActor can be used to set a TID on an actor under the player's crosshair (or using any TID as the source really). Filtering can be done on the actor flags level.

PickActor(tid, angle, pitch, max_distance, set_tid, [actormask], [wallmask])

A corresponding pull request exists for the function definition and wallmask constants in ACC.