Add support for autonomous lua entities

Autonomous entities remain active at all times, keeping a radius of
blocks around them active in the same way a player does (although with a
different and configurable radius to that used for players).

A lua entity can set itself into autonomous mode via the api, and should
switch it off again when it no longer needs it.

Autonomous capability is completely disabled by default and can only be
enabled by a setting in minetest.conf.