NAME
        CMD_STOP -- pause current and queued I/O requests (immediate)

   FUNCTION
        CMD_STOP pauses all queued requests for the unit, and tries to
        pause the current I/O request.  The only commands that will
        be subsequently allowed to be performed are immediate I/O
        requests, which include those to start, flush, and finish the
        I/O after the stop command.

   IO REQUEST
        io_Message      mn_ReplyPort set if quick I/O is not possible
        io_Device       preset by the call to opendevice
        io_Command      CMD_STOP
        io_Flags        IOB_QUICK set if quick I/O is possible