Fixes slipping on water while riding janicart.

The handle_slip() now also handles cases where the mob is buckled. If you are buckled and slip on lube (or something that ignores

galoshes) you get unbuckled and stunned and the buckle object moves one step (no slides unlike when unbuckled).
You can thus slip on lube while buckled to a chair or a roller bed.
Fixes getting hit by abstract held items when slipping.
Fixes janicart rider getting hit twice by bullets.
Removing the STEP constant var in slipping code, it's no more used.
Fixes carbon/slip() not returning the correct value.
Changed var/obj/structure/stool/bed/buckled to var/obj/buckled since buckled can now technically be any obj.

Removes hacky unbuckling/buckling in bed/Move()

code/game/objects/structures/janicart.dm

@@ -238,14 +238,6 @@
 		step(src, direction)
 		update_mob()
 		handle_rotation()
-		if(istype(src.loc, /turf/simulated))
-			var/turf/simulated/T = src.loc
-			if(T.wet == 2)	//Lube! Fall off!
-				playsound(src, 'sound/misc/slip.ogg', 50, 1, -3)
-				buckled_mob.Stun(7)
-				buckled_mob.Weaken(7)
-				unbuckle_mob()
-				step(src, dir)
 		move_delay = 1
 		spawn(2)
 			move_delay = 0
@@ -300,11 +292,6 @@
 				buckled_mob.pixel_x = -12
 				buckled_mob.pixel_y = 7
 
-
-/obj/structure/stool/bed/chair/janicart/bullet_act(obj/item/projectile/Proj)
-	if(buckled_mob)
-		buckled_mob.bullet_act(Proj)
-
 /obj/item/key
 	name = "key"
 	desc = "A small grey key."

code/game/objects/structures/stool_bed_chair_nest/bed.dm

@@ -24,14 +24,12 @@
 /obj/structure/stool/bed/Move(atom/newloc, direct) //Some bed children move
 	. = ..()
 	if(buckled_mob)
-		buckled_mob.buckled = null
 		if(!buckled_mob.Move(loc, direct))
 			loc = buckled_mob.loc //we gotta go back
 			last_move = buckled_mob.last_move
 			inertia_dir = last_move
 			buckled_mob.inertia_dir = last_move
 			. = 0
-		buckled_mob.buckled = src
 
 /obj/structure/stool/bed/Process_Spacemove(movement_dir = 0)
 	if(buckled_mob)