INT 2F - DOS 4+ APPEND - GET APPEND FUNCTION STATE
        AX = B706h
Return: BX = APPEND state
            bit 0: set if APPEND enabled
            bits 1-11 reserved
            bit 12: (DOS 5.0) set if APPEND applies directory search even if a
                        drive has been specified
            bit 13: set if /PATH flag active
            bit 14: set if /E flag active (environment var APPEND exists)
            bit 15: set if /X flag active