INT 6F - 10-NET - UNLOCK HANDLE
        AH = 08h
        BX = file handle
        AL = mode
            00h unlock all
            01h unlock record at CX:DX
Return: CF set on error
            AX = error code (see also AH=00h)
                0002h file not found
        CF clear if successful
SeeAlso: INT 21/AH=5Ch