Make rails selectable.

Rails weren't selectable, and couldn't be removed from a map asides from
using worldedit (or maybe sand)

mods/default/init.lua

@@ -939,7 +939,8 @@ minetest.register_node("default:rail", {
 	walkable = false,
 	selection_box = {
 		type = "fixed",
-		--fixed = <default>
+                -- but how to specify the dimensions for curved and sideways rails?
+                fixed = {-1/2, -1/2, -1/2, 1/2, -1/2+1/16, 1/2},
 	},
 	groups = {bendy=2,snappy=1,dig_immediate=2},
 })