Conductive

Makes non-standard rail conductive, so there doesn't have to be a mesecon line right next to it. Four-ways can now be powered without having a mesecon above/below it.
Depends on https://github.com/minetest/minetest/pull/473 (rail disconnects from itself while turning on/off without this)
Depends on https://github.com/PilzAdam/carts/pull/5 (couldn't get these two commits separated)