This allows mod makers to register their own recipies, including
none, to register doors.

This patch replaces the default door nodes with a new mesh model
and nodes.

Two new models were added that are 2 blocks high. One for left-hinge
and one for right-hinge doors. This allows us to make a single texture
fit on both models. The alternative would have been 1 model and 2
unmapped textures, which is more work for mod developers.

Doors work exactly like the old doors, including ownership, breaking
doors, opening and closing.

Under the hood, we can prevent the top part of the door from being
obstructed by placing an invisible node. This prevents liquids from
flowing through doors or people placing sand or other blocks in the
top half. The door code automatically places and removes these as
needed.

Metadata is used to store door state, just like the old version.

A doors API is added, it allows mods to use the API to open/close or
toggle door states without worrying about sounds, permissions and
other details. This is intended for e.g. mesecons. This API allows
mods to manipulate or inspect doors for players or for themselves.

In-game old door nodes are automatically converted using an ABM and
preserve ownership and orientation and state.

TNT blows up all doors and trapdoors except for the steel ones,
who can survive a blast. We return an itemstack in on_blast(),
which requires a TNT API patch which is also pending.

We enable backface culling for most of these doors, as this gives
the identical visual appearance that the old doors had. In the case
of the glass door, there's a slight twist.

The texture files used by the new doors have new names that do
not conflict with previous texture file names to avoid texture
pack conflicts.

Thanks to red-001 <red-001@users.noreply.github.com> for some
of the conversion code, cleanups, and extra textures.

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.

Rename in game.conf and documentation
Update game_api.txt documentation for bucket API and tree functions
Fix tab, space and comment formatting in game_api.txt
Rename in mod READMEs

Uses group “connect_to_raillike”

Used value: 2

- Adds material for crafting (keeps def.recipe compatibility)
- Fix two typos in api documentation

can be registered by other mods