Previously, opening a trapdoor caused the side textures to flip.

Fix the incorrect textures.
Also add a texture transform to a tile of the open trapdoor, such
that the closed trapdoor sides use the lower part of the texture
and the open trapdoor sides use the higher part.

Clean up some codestyle issues.

Allow mods such as protection mods to over ride on_rightclick.
Usecase is creating shared doors without the need for keys.

Forgot this in the last commit, thanks @Fixer-007

Due to door items being the thing that's placed, we need to
explicitly play a sound when placing a door.

Fixes #1363

As written in game_api.txt.

Seems like this was a typo

Fixes bug caused by b847888c

Completes a forgotten update in 9d3a5263

Utilizes several new features allowing the description of an item to be changed using the `description` meta key. This also moves keys from using the old single-value itemstack metadata system to the new node-like metadata system.

This reverts commit 5e4a6e8a, and
             commit 60cf3f85.

The original door API rewrite I posted had all rotation disabled using
the correct on_rotate() handler returning `false`. Two subsequent
changes attempting first to enable simple rotation and second disabling
that again changed the callback handler to a boolean `false`, which
is incorrect use in the screwdriver API, and actually allows rotation.

The proper way to disable rotation for facedir nodes it to use a full
callback handler that does `return false`.

Fixes #1364

This code adds the key concept to minetest_game, and integrates it
with lockable nodes. Currently supported lockable items are the Steel
Door, the Steel Trapdoor, and the Locked Chest.

The goal of this modification is to introduce a fine-grained multi-
player permission system that is intuitive and usable without any
console or chat commands, and doesn't require extra privileges to
be granted or setup. Keys can also physically be conveyed to other
players, adding to gameplay and adding some personality that is
preferable to console commands or editing formspecs.

A skeleton key can be crafted with 1 gold ingot. Skeleton keys can
then be matched to a lockable node by right-clicking the skeleton
key on a lockable node, which changes the skeleton key to a "key".

Gold was chosen as it's currently a not-so very useful item, and
therefore it's likely that players have some, but aren't really
using it for any purpose.

This key can subsequently used by any player to open or access that
lockable node, including retrieving items from Locked Chests, or
putting items in them.

They key is programmed to fit only the particular locked node it is
programmed to. This is achieved by storing a secret value in both
key and locked node. If this secret value doesn't match, the key
will not open the locked node. This allows many keys to be created
for one chest or door, but a key will only fit one node ever. The
secrets are stored in node, and item meta for the key.

If a locked node is removed, all keys that opened it are no longer
valid. Even if a new door/chest is placed in exactly the same spot,
the old keys will no longer fit that node.

Keys can be smelted back in gold ingots if they are no longer useful.

The method of storing a secret in nodemeta and itemstackmeta is secure
as there is no way for the client to create new items on the server
with a particular secret metadata value. Even if you could possible
create such an itemstack on the client, the server does not ever read
itemstackmeta from a client package.

The patch adds an API that allows other nodes and nodes added by
mods to use the same keys as well. The method how to implement this
is described in game_api.txt. The mod should add 2 callbacks to it's
node definition. Example code is given.

Textures are from PixelBOX, thanks to Gambit.

Currently all minetest_game PR's are failing travis since the
upstream luacheck now also warns about whitespace issues, and
there are a few of those in the code. This fixes all of them
so we can yet again rely on luacheck.

Edited fuels: fences, wooden ladder.
New fuels:
boat, paper, book, dry shrub, stick, vessels shelf, wooden tools,
wooden door, trapdoor, fence gates, farming crops, stairs, slabs, beds.

Rotating doors with screwdrivers causes too many issues to be worth it.

The collision box still extended into a neighbouring empty node, causing
falling node objects to collide but not transform back into normal nodes.
Completes the fix started in a previous similar commit.

Make the softer woods, pine and aspen, 'flammable = 3'.
Correct inconsistent flammability of wood and stairs in relation
to all other solid wood nodes in MTGame.
Make the the softer woods, pine and aspen, 'choppy = 3'.

Keep other axis' disabled to prevent the hidden placeholder node to become irremovable to players.

Previously, the collision box extended into an empty node, causing
falling node objects to land on the open gate but not transform
back into normal nodes. Now fallng node objects will fall through and
either side of the end of the open gate and transform back.

Fix crash when doors are placed under unknown nodes.
Share a can_dig among doors, that does not crash on nil-player.
Only set can_dig if we actually protect the door.

Removing the node above was an error and is unnecessary