Automatically board and unboard vehicles.

* Separates the idea of vehicle control from being inside.
* No more "Board Y/N" prompt.
* Players can clamber around inside moving vehicles if they want.
* Boardable parts include seats, beds, boxes, aisles and doors.
  If the player is on any of these, they will be carried with the vehicle.
* Player can dismount by walking out of the vehicle.
* Suicidally moving onto moving frames etc is disallowed.

I didn't actually check, but this almost certainly breaks save compatibility.

Invalidates #1484 and #1516.

I didn't touch cruise control or skidding behaviour. Vehicles start skidding and slowing almost as soon as you let go of the wheel.

Acquiring zombie passengers is still, sadly, impossible.