Implement drag force according to the equation:
drag_force = drag_coefficient * speed ^2
Also add a small constant force to implement friction force.

These are unrelated to the Player API and player object.
All other GUI/HUD code and textures are in default.
All other engine hardcoded sounds are in default.
The player_api mod is highly likely to be used unchanged in new games,
so logical grouping of content will help the creation of new games.

No longer use the unsuitable steel door/trapdoor sound.

Avoids player-placed leaves decaying when building while sneaking, which is common.

Register using the 'doors' mod API.

Increase noise resolution from 4 to 2 nodes for a higher quality
distribution.
Retune noise parameters to compensate for using eased noise.

Add an option, default disabled, to extend the collision boxes upwards.

broken by bb9279cc

default:river_water_source can also create sound if desired as
rivers are considered to be flowing water.
A simple mod for now, with the intention to later use new engine
environmental sound features if/when they appear.

rebased #2368 

Allows the initial hand registration to alter the 'wield_scale' without
needing to also alter it in creative mod.
Also make default mod a dependency again, as the initial hand
registration is required.

Previously, the value of 'mapgen_limit' was not used to limit the
spawn position.
If a friendly-biome spawn point is not found within a small world,
spawn point falls back to the engine spawn point, which has a larger
chance of success.

The output returned by `get_craft_result` has a replacements table that
contains any replacement items that could not be placed in the input
(due to the input stack having a count > 1). Put these replacements in
the dst list or drop them on the furnace.

This prevents a crash when a 'nil' digger is passed by the engine to
minetest.node_dig.