INT 2F U - NETWORK REDIRECTOR (DOS 3.x only) - UNLOCK REGION OF FILE
        AX = 110Bh
        BX = file handle
        CX:DX = starting offset
        SI = high word of size
        STACK: WORD low word of size
        ES:DI -> SFT for file
            SFT DPB field -> DPB of drive containing file
Return: CF set on error
           AL = DOS error code (see INT 21/AH=59h)
        STACK unchanged
Note:   called by DOS 3.1-3.31 kernel; DOS 4.0+ calls AX=110Ah instead
SeeAlso: INT 21/AH=5Ch