Further controls refactoring (keyboard only)

Keys are now separate,
Refactored out into scripts.
Next step is gamepads!

Further refactored input checking code so it is separate from raw
keyboard_check_* functions. This paves the way for both gamepads and
control-based AI. Also, fixed the "always dropping" bug. (It was likely
caused by a frame-skip so keyboard_check_released didn't ever trigger to
turn off the drop.)