INT 2F U - IFSFUNC.EXE (DOS 4.x only) - ???
        AX = 112Dh
        BL = subfunction (value of AL on INT 21)
            04h truncate open file to zero length
                ES:DI -> SFT for file
                Return: CF clear
            else ???
                Return: CX = ??? (00h or 02h for DOS 4.01)
        ES:DI -> SFT
        SS = DOS DS
Return: DS = DOS DS
Note:   called by DOS 4.0 kernel on INT 21/AX=5702h,INT 21/AX=5703h,
          INT 21/AX=5704h