INT 2F U - DOS 3+ internal - SIGNAL SHARING VIOLATION TO USER
        AX = 120Bh
        ES:DI -> system file table entry for previous open of file
        STACK: WORD extended error code (should be 20h--sharing violation)
Return: CF clear if operation should be retried
        CF set if operation should not be retried
            AX = error code (20h) (see INT 21/AH=59h)
        STACK unchanged
Notes:  can only be called during a DOS function call
        should only be called if an attempt was made to open an already-open
          file contrary to the sharing rules
        invokes INT 24 if SFT file opened via FCB or in compatibility mode with
          inheritance allowed