Register only locked nodes as stoppers

This was suggested here:
https://github.com/Jeija/minetest-mod-mesecons/pull/216

mesecons_mvps/init.lua

@@ -231,7 +231,8 @@ function mesecon.mvps_move_objects(pos, dir, nodestack)
 	end
 end
 
+mesecon.register_mvps_stopper("doors:door_steel_b_1")
+mesecon.register_mvps_stopper("doors:door_steel_t_1")
+mesecon.register_mvps_stopper("doors:door_steel_b_2")
+mesecon.register_mvps_stopper("doors:door_steel_t_2")
 mesecon.register_mvps_stopper("default:chest_locked")
-mesecon.register_mvps_stopper("default:chest")
-mesecon.register_mvps_stopper("default:bookshelf")
-mesecon.register_mvps_stopper("default:furnace")