*_move functions now do not affect monster plans and renamed sound_move to wander_next.

sound_move wasn't a very good name, as it's effectively just "wander towards X", and might be used by other things than sound in the future.

Doing plans.clear() in the *_move() functions made it hard to use these functions outside the environment they are currently being called in. By having them as pure lookup algorithms, these functions are now much more flexible.