Fix `maptools:drowning` using an invalid value and being walkable

This prevented Minetest from starting (tested on
Minetest 5.3.0-dev-cb9a44ef8).

Being walkable also made it impossible to actually drown in, except
when using noclip mode.

nodes.lua

@@ -476,7 +476,8 @@ minetest.register_node("maptools:drowning", {
 	drawtype = "airlike",
 	paramtype = "light",
 	pointable = false,
-	drowning = true,
+	walkable = false,
+	drowning = 1,
 	sunlight_propagates = true,
 	drop = "",
 	groups = {unbreakable = 1, not_in_creative_inventory = maptools.creative},