Document input routines.
Group key_defs.
Start documenting nighttime and spotlight code.
Start documenting called_from_main_loop_7.
Start documenting move_characters.
Drop duplicate counter_of_something.
Document counter_set_0 which seems to be a static tile plotter.
Note the flags returned from user_confirm.
Use 'nonzero' rather than 'NZ' for consistency.