INT 6F - 10-NET - LOCK HANDLE
        AH = 07h
        BX = file handle
        CX:DX = starting offset in file
        SI = record length
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