Used by the routines at TAKE_OR_LEAVE, BOLT, HOMER, PUSHABLE, BOBBER, SPIDER, WELL, ROAMER and SKY_WALKER.

Takes one off dZ -- the pull of gravity -- then lets ADJ_FOR_OUT_OF_BOUNDS cut the step down to what fits, and adds what is left to the position. Most update routines come in at CLIP_AND_MOVE, to move without falling; the player's legs come in at ADD_DUVZ, having cut the move themselves (MOVE_PLAYER).