- added GetActorSectorFloorZ and GetActorSectorCeilingZ ACS functions (take 2)

from here: http://forum.zdoom.org/viewtopic.php?f=3&t=45543

These functions return floor and ceiling z by actor's sector, in contrast with GetActorFloorZ and GetActorCeilingZ which returns the actor's floorz and ceilingz fields. These versions are compatible with GetActorLightLevel, CheckActorFloorTexture, CheckActorCeilingTexture, maybe some others that I don't know yet of either..

It's 2014, let's make ZDoom ACS able to check whether the player is in lava or not. :cactus: