INT 21 - LANtastic - SET MESSAGE PROCESSING FLAGS
        AX = 5F9Bh
        DL = bits describing processing for received unsolicited messages
             (see AX=5F9Ah)
Return: CF clear if successful
        CF set on error
            AX = error code
SeeAlso: AX=5F9Ah,AX=5F9Eh

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