Refactor non-portable pointer manipulation routines.
Move DOS register types into a common directory.
PLAY: STATIC_ASSERT structure sizes.
PLAY: use WORD (2 bytes) and LONG (4 bytes) for portability.
PLAY: fix type of frame_val.
PLAY: replace VGA_SCREEN with vf.p.
PLAY: replace fclose with gentle_close.
Move some I/O routines into a common directory.
PLAY: refactor file system routines.
PLAY: SDL port.
PLAY: fix flic menu group and identity types.
PLAY: fix sign compare warnings.