Fixed walking up stairs.

* Now they won't try and bump you up to the next z on the "lower" step of the stairway.  They also don't directly set `loc`  Therefore pulling objects up/down actually works.
* Sets their layer so they won't appear above actual objects and mobs.

code/modules/multiz/structures.dm

@@ -129,6 +129,7 @@
 	density = 0
 	opacity = 0
 	anchored = 1
+	layer = 2.4 // Above turf, but they're sort of the floor, so below objects.
 
 /obj/structure/stairs/initialize()
 	for(var/turf/turf in locs)
@@ -140,13 +141,12 @@
 			above.ChangeTurf(/turf/simulated/open)
 
 /obj/structure/stairs/Uncross(atom/movable/A)
-	if(A.dir == dir)
+	if(A.dir == dir && upperStep(A.loc))
 		// This is hackish but whatever.
 		var/turf/target = get_step(GetAbove(A), dir)
 		var/turf/source = A.loc
 		if(target.Enter(A, source))
-			A.loc = target
-			target.Entered(A, source)
+			A.forceMove(target)
 			if(isliving(A))
 				var/mob/living/L = A
 				if(L.pulling)
@@ -154,6 +154,9 @@
 		return 0
 	return 1
 
+/obj/structure/stairs/proc/upperStep(var/turf/T)
+	return (T == loc)
+
 /obj/structure/stairs/CanPass(obj/mover, turf/source, height, airflow)
 	return airflow || !density