Improved player physics

This contains several changes to player physics, meant to fix important flaws with basic movement. Main features are:

- Movement settings are now part of minetest.conf, and the server parses them to clients upon connecting. Each server can have custom movement / acceleration parameters, water friction levels, and even low gravity is possible.

- Air acceleration has been reduced to 1/4 of ground acceleration by default. Previously you could "walk on air" which felt very wrong and unrealistic.

- Adaptive liquid friction. Players can now dive in water and will sink based on the velocity they hit the surface with. Previously, touching water made the player's velocity magically disappear.

- Better swimming mechanics. Fast movement can be used underwater, and players now use the Sneak key to sink (there's still a small amount of automatic sinking as well).

- Vertical movement uses acceleration and works the same as horizontal movement.