Dedicated fence rail textures coming in a follow-up PR.

the ```group:cracky``` group contains all sorts of odds and ends
nodes that we shouldn't connect to. There are a few nodes that
walls now no longer connect to, that probably should get
```group:stone``` added, or something similar, though.

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.