Basic stone walls, using NDT_CONNECTED.

These basic connected wall nodes automatically connect
to neigboring stone blocks, other wall blocks and anything
that's "cracky". The do not connect to wood (fences will do
that).

The walls are generated using a new walls.register() API.
Documentation on the API is included in game_api.txt.

This change requires minetest/minetest#3503.

Walls are added for all cobble stone materials. They generally
look best and are the natural use cases for these materials.

game_api.txt

@@ -139,6 +139,17 @@ Allows creation of new fences with "fencelike" drawtype.
 	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
 	sounds = default.node_sound_wood_defaults(),
 
+#Walls API
+---------
+The walls API allows easy addition of stone auto-connecting wall nodes.
+
+walls.register(name, desc, texture, mat, sounds)
+^ name = "walls:stone_wall". Node name.
+^ desc = "A Stone wall"
+^ texture = "default_stone.png"
+^ mat = "default:stone". Used to auto-generate crafting recipe.
+^ sounds = sounds: see [#Default sounds]
+
 Farming API
 -----------