NAME
        IND_ADDHANDLER -- Add an input handler to the device

   FUNCTION
        Add a function to the list of functions called to handle
        input events generated by this device.  The function is called
        as
            newInputEvents = Handler(inputEvents, handlerData);
            D0                       A0           A1

   IO REQUEST
        io_Message      mn_ReplyPort set
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      IND_ADDHANDLER
        io_Data         a pointer to an interrupt structure.
            is_Data     the handlerData pointer described above
            is_Code     the Handler function address

   NOTES
        The interrupt structure is kept by the input device until a
        RemHandler command is satisfied for it.