Replace flag constants with symbolic equivalents.
character_sits: Replace numbers with constants.
Rationalise flag symbol names.

Note: committing out of order.
Change the 'divide AC' routines to not specify a register but instead
use a fake argument list. It was wrong anyway and should have been 'CA'.
map_position_maybe -> map_position
Rework called_from_main_loop_5 and process_user_input.
Break up plot_game_screen. Resolve banked registers.
Masked sprite plotting routines. Only partly complete.