NAME
        CMD_CLEAR -- Clear the keyboard input buffer.

   FUNCTION
        remove from the input buffer any keys transitions waiting to
        satisfy read requests.

   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_CLEAR
        io_Flags        IOB_QUICK set if quick I/O is possible