Split vehicle control out of '$' onto its own keybinding (^), and add Exit Vehicle to the menu.

I wanted to move vehicle exiting to the vehicle control menu, as suggested by TheDarklingWolf in issue #1279, but overloading ACTION_SLEEP was getting unwieldy. I think vehicle control deserves its own keybinding anyway. '^' was unused.

Changes:
* Split vehicle disembarking out of game::examine() into game::exit_vehicle().
* Split vehicle control out of ACTION_SLEEP into ACTION_CONTROL_VEHICLE.
* Add new default keybinding for vehicle control: '^'.
* Allow vehicle disembark from vehicle control menu.
* Only query for disembarking moving vehicles.
* Properly fling the player from moving vehicles when they do decide to exit them.