placates gcc which seems to like to complain about -1 being assigned to
an unsigned value. It is well defined and intended, but since signess bugs
are being hunted just change to 0xffffffff.
o Mask the lower 8 bits, not the lower 4 bits for the ai_capabilities word.
All 8 bits are defined and the 0xf was almost certainly a typo.
o Define APM_UNKNOWN to 0xff for emulation layer.