Create API for fence.register, and use it.

This converts the call to minetest.register() for the default
fence node, so it can be called by other mods to quickly
setup other fences.

Since this creates an API, insert it into the game_api.txt.

The api looks like minetest.register(name, {def}), and has two
uncommon fields: "texture" and "material". Any normal nodedef
property can be passed through, except "drawtype". The "fence"
group will always be added.

The default fence recipe is modified to be as follows:
  wood, stick, wood
  wood, stick, wood

This recipe yields 4 fence nodes.

This allows us to create according recipes for acacia, pine,
aspen, and junglewood fences without adding new stick types:

  pine wood, stick, pine wood
  pine wood, stick, pine wood

This is a from-scratch implementation, written by heart but inspired
by (#665 - Add many wooden fences).

Stick and fences nodes are named in a consistent way.

game_api.txt

@@ -110,6 +110,24 @@ doors.register_trapdoor(name, def)
 		will be overwritten by the trapdoor registration function
 }
 
+Fence API
+---------
+Allows creation of new fences with "fencelike" drawtype.
+
+default.register_fence(name, item definition)
+ ^ Registers a new fence. Custom fields texture and material are required, as
+ ^ are name and description. The rest is optional. You can pass most normal
+ ^ nodedef fields here except drawtype. The fence group will always be added
+ ^ for this node.
+
+#fence definition
+	name = "default:fence_wood",
+	description = "Wooden Fence",
+	texture = "default_wood.png",
+	material = "default:wood",
+	groups = {choppy=2, oddly_breakable_by_hand = 2, flammable = 2},
+	sounds = default.node_sound_wood_defaults(),
+
 Farming API
 -----------
 The farming API allows you to easily register plants and hoes.

mods/default/crafting.lua

@@ -42,14 +42,6 @@ minetest.register_craft({
 	}
 })
 
-minetest.register_craft({
-	output = 'default:fence_wood 2',
-	recipe = {
-		{'group:stick', 'group:stick', 'group:stick'},
-		{'group:stick', 'group:stick', 'group:stick'},
-	}
-})
-
 minetest.register_craft({
 	output = 'default:sign_wall',
 	recipe = {

mods/default/functions.lua

@@ -209,6 +209,51 @@ function default.dig_up(pos, node, digger)
 end
 
 
+--
+-- Fence registration helper
+--
+function default.register_fence(name, def)
+	minetest.register_craft({
+		output = name .. " 4",
+		recipe = {
+			{ def.material, 'group:stick', def.material },
+			{ def.material, 'group:stick', def.material },
+		}
+	})
+
+	local fence_texture = "default_fence_overlay.png^" .. def.texture ..
+			"^default_fence_overlay.png^[makealpha:255,126,126"
+	-- Allow almost everything to be overridden
+	local default_fields = {
+		paramtype = "light",
+		drawtype = "fencelike",
+		inventory_image = fence_texture,
+		wield_image = fence_texture,
+		tiles = { def.texture },
+		sunlight_propagates = true,
+		is_ground_content = false,
+		selection_box = {
+			type = "fixed",
+			fixed = {-1/7, -1/2, -1/7, 1/7, 1/2, 1/7},
+		},
+		groups = {},
+	}
+	for k, v in pairs(default_fields) do
+		if not def[k] then
+			def[k] = v
+		end
+	end
+
+	-- Always add to the fence group, even if no group provided
+	def.groups.fence = 1
+
+	def.texture = nil
+	def.material = nil
+
+	minetest.register_node(name, def)
+end
+
+
 --
 -- Leafdecay
 --

mods/default/nodes.lua

@@ -151,6 +151,10 @@ default:sign_wall
 default:ladder
 
 default:fence_wood
+default:fence_acacia_wood
+default:fence_junglewood
+default:fence_pine_wood
+default:fence_aspen_wood
 
 default:glass
 default:obsidian_glass
@@ -1674,26 +1678,45 @@ minetest.register_node("default:ladder", {
 	sounds = default.node_sound_wood_defaults(),
 })
 
-
-local fence_texture =
-	"default_fence_overlay.png^default_wood.png^default_fence_overlay.png^[makealpha:255,126,126"
-minetest.register_node("default:fence_wood", {
+default.register_fence("default:fence_wood", {
 	description = "Wooden Fence",
-	drawtype = "fencelike",
-	tiles = {"default_wood.png"},
-	inventory_image = fence_texture,
-	wield_image = fence_texture,
-	paramtype = "light",
-	sunlight_propagates = true,
-	is_ground_content = false,
-	selection_box = {
-		type = "fixed",
-		fixed = {-1/7, -1/2, -1/7, 1/7, 1/2, 1/7},
-	},
+	texture = "default_wood.png",
+	material = "default:wood",
 	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
-	sounds = default.node_sound_wood_defaults(),
+	sounds = default.node_sound_wood_defaults()
+})
+
+default.register_fence("default:fence_acacia_wood", {
+	description = "Acacia Fence",
+	texture = "default_acacia_wood.png",
+	material = "default:acacia_wood",
+	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
+	sounds = default.node_sound_wood_defaults()
+})
+
+default.register_fence("default:fence_junglewood", {
+	description = "Junglewood Fence",
+	texture = "default_junglewood.png",
+	material = "default:junglewood",
+	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
+	sounds = default.node_sound_wood_defaults()
 })
 
+default.register_fence("default:fence_pine_wood", {
+	description = "Pine Fence",
+	texture = "default_pine_wood.png",
+	material = "default:pine_wood",
+	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
+	sounds = default.node_sound_wood_defaults()
+})
+
+default.register_fence("default:fence_aspen_wood", {
+	description = "Aspen Fence",
+	texture = "default_aspen_wood.png",
+	material = "default:aspen_wood",
+	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
+	sounds = default.node_sound_wood_defaults()
+})
 
 minetest.register_node("default:glass", {
 	description = "Glass",