add utility alias for level-0 leveled nodes.

leveled nodes now have an alias from level 0 to air, meaning that code
that changes the level of a node doesn't have to differentiate between
setting its level, and removing the node when the level is supposed to
be 0.

mods/bg_api/leveled_node.lua

@@ -79,4 +79,5 @@ function blockgame.register_leveled_node (name, def)
 	end
 
 	minetest.register_alias(fullname, fullname .. "_" .. def.level_max)
+	minetest.register_alias(fullname .. "_" .. 0, "air")
 end