zero cart_acceleration

On regular rail, a pushed cart will slow down and stop, then move back slightly. On unpowered rail, (with `minetest.env:get_meta(pos):set_string("cart_acceleration", "0"`) the cart will slow down, and go forward and back in a manner that it seems the forward and back motions are fighting with each other. This is one way of making the cart come to a full stop on rail that has zero acceleration set, the same as it stops on rail that has no acceleration set.