INT 2F - DESQview v2.26+ XDI - CUSTOM SUBFUNCTION, Ralf Brown's XDI drivers
        AX = DE01h
        BX = 5242h ("RB")
        CX:DX = program identifier
                656F7000h ("eop",0) for DVeop
Return: AX = 5242h ("RB") if installed
            ES:BX -> data or entry point
            CX = version number (CH = major, CL = minor)

Call DVeop entry point with:
        ES:DI -> callback address or 0000h:0000h to remove callback
Return: AX = status
            0000h failed (callback table full or attempted to remove non-
                        existent callback)
            0001h successful
                ES:DI -> chaining address
        BX,CX,DX destroyed
Notes:  the callback function is called with a simulated interrupt when the
          DESQview window containing it is closed; it should perform all
          necessary cleanup and then perform a FAR jump to the chaining address
          or an IRET if the chaining address is 0000h:0000h
        if the program wishes to remove itself before the window is closed, it
          should call the DVeop entry point with the previously returned
          chaining address and ignore the returned chaining address.