INT 6F - 10-NET - UNLOCK FCB
        AH = 12h
        AL = mode
            00h sequential
            01h random
            02h random block
                CX = number of records
        DS:DX -> FCB (see INT 21/AH=0Fh)
Return: CF set on error
            AX = error code (see also AH=00h)
                0002h file not found
        CF clear if successful