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