INT 2F U - DOS 3.3+ DISPLAY.SYS internal - INSTALLATION CHECK
        AX = AD00h
Return: AL = FFh if installed
            BX = ??? (0100h in MS-DOS 3.30, PCDOS 4.01)
Note:   DOS 5.0 DISPLAY.SYS chains to previous handler if AL is not one of the
          subfunctions listed here