Make avatar respect maxspeed lerp, allow changing speed multiplier via script. Some extra functions.

Add one additional not-timed speed multiplier that also ignores influence by currents.
Lua functions added:
- avatar_setSpeedMult(num, time)
- avatar_setSpeedMult2(num)
- avatar_getSpeedMult()
- avatar_getSpeedMult2()
- entity_isInCurrent(entity)
- entity_getNumTargetPoints(entity)