INT 6F - 10-NET v3.3 - UNMOUNT
        AH = 18h
        AL = local drive number (0=A:)
        BL = type
            00h     disk
            01h-03h LPTn
            04h,05h COMx
Return: CF set on error
            AX = error code (see AH=00h)
        CF clear if successful