NAME
       flush -- clear all queued i/o requests for the serial port

   FUNCTION
       This command purges the read and write request queues for the
       serial device. flush will not affect active requests.

   IO REQUEST
       io_Message      mn_ReplyPort initialized
       io_Device       set by opendevice
       io_Unit         set by opendevice
       io_Command      CMD_FLUSH

   RESULTS
       Error -- if the flush succeded, then io_error will be null.
                 If the flush failed, then the io_error will be non-zero.