Default: Generalise, optimise and simplify grass spread function

Credit to tenplus1 for the suggestion to generalise for mod use.
Mods can add mod nodes to 'group:spreading_dirt_type' enabling the
function to work with mod nodes.

Add some nodes to this group.

Removing 'dirt_with_grass' etc. from 'neighbors' stops the ABM action
running everywhere and constantly, on the dirt nodes immediately below
the surface nodes. Now the action only runs in the rare case of a dirt
node with neighbouring air, grass decorations or snow.

Remove check for air above to allow grass to spread under light-
transmitting nodes such as fences, walls, plants. This causes spread
under slabs, stairs and glass, when near air, but seems worth it.

Remove unnecessary check for nil node.

mods/default/functions.lua

@@ -348,9 +348,7 @@ minetest.register_abm({
 	label = "Grass spread",
 	nodenames = {"default:dirt"},
 	neighbors = {
-		"default:dirt_with_grass",
-		"default:dirt_with_dry_grass",
-		"default:dirt_with_snow",
+		"air",
 		"group:grass",
 		"group:dry_grass",
 		"default:snow",
@@ -359,36 +357,27 @@ minetest.register_abm({
 	chance = 67,
 	catch_up = false,
 	action = function(pos, node)
-		-- Most likely case, half the time it's too dark for this.
+		-- Check for darkness: night, shadow or under a light-blocking node
+		-- Returns if ignore above
 		local above = {x = pos.x, y = pos.y + 1, z = pos.z}
 		if (minetest.get_node_light(above) or 0) < 13 then
 			return
 		end
 
-		-- Look for likely neighbors.
-		local p2 = minetest.find_node_near(pos, 1, {"default:dirt_with_grass",
-				"default:dirt_with_dry_grass", "default:dirt_with_snow"})
+		-- Look for spreading dirt-type neighbours
+		local p2 = minetest.find_node_near(pos, 1, "group:spreading_dirt_type")
 		if p2 then
-			-- But the node needs to be under air in this case.
-			local n2 = minetest.get_node(above)
-			if n2 and n2.name == "air" then
-				local n3 = minetest.get_node(p2)
-				minetest.set_node(pos, {name = n3.name})
-				return
-			end
-		end
-
-		-- Anything on top?
-		local n2 = minetest.get_node(above)
-		if not n2 then
+			local n3 = minetest.get_node(p2)
+			minetest.set_node(pos, {name = n3.name})
 			return
 		end
 
-		local name = n2.name
-		-- Snow check is cheapest, so comes first.
+		-- Else, any seeding nodes on top?
+		local name = minetest.get_node(above).name
+		-- Snow check is cheapest, so comes first
 		if name == "default:snow" then
 			minetest.set_node(pos, {name = "default:dirt_with_snow"})
-		-- Most likely case first.
+		-- Most likely case first
 		elseif minetest.get_item_group(name, "grass") ~= 0 then
 			minetest.set_node(pos, {name = "default:dirt_with_grass"})
 		elseif minetest.get_item_group(name, "dry_grass") ~= 0 then
@@ -404,11 +393,7 @@ minetest.register_abm({
 
 minetest.register_abm({
 	label = "Grass covered",
-	nodenames = {
-		"default:dirt_with_grass",
-		"default:dirt_with_dry_grass",
-		"default:dirt_with_snow",
-	},
+	nodenames = {"group:spreading_dirt_type"},
 	interval = 8,
 	chance = 50,
 	catch_up = false,

mods/default/nodes.lua

@@ -332,7 +332,7 @@ minetest.register_node("default:dirt_with_grass", {
 	tiles = {"default_grass.png", "default_dirt.png",
 		{name = "default_dirt.png^default_grass_side.png",
 			tileable_vertical = false}},
-	groups = {crumbly = 3, soil = 1},
+	groups = {crumbly = 3, soil = 1, spreading_dirt_type = 1},
 	drop = 'default:dirt',
 	sounds = default.node_sound_dirt_defaults({
 		footstep = {name = "default_grass_footstep", gain = 0.25},
@@ -357,7 +357,7 @@ minetest.register_node("default:dirt_with_dry_grass", {
 		"default_dirt.png",
 		{name = "default_dirt.png^default_dry_grass_side.png",
 			tileable_vertical = false}},
-	groups = {crumbly = 3, soil = 1},
+	groups = {crumbly = 3, soil = 1, spreading_dirt_type = 1},
 	drop = 'default:dirt',
 	sounds = default.node_sound_dirt_defaults({
 		footstep = {name = "default_grass_footstep", gain = 0.4},
@@ -369,7 +369,7 @@ minetest.register_node("default:dirt_with_snow", {
 	tiles = {"default_snow.png", "default_dirt.png",
 		{name = "default_dirt.png^default_snow_side.png",
 			tileable_vertical = false}},
-	groups = {crumbly = 3, soil = 1},
+	groups = {crumbly = 3, soil = 1, spreading_dirt_type = 1},
 	drop = 'default:dirt',
 	sounds = default.node_sound_dirt_defaults({
 		footstep = {name = "default_snow_footstep", gain = 0.15},