INT 14 - MultiDOS Plus - SET MONITOR MODE
        AH = 24h
        AL = port status storage
            00h single status for entire receive buffer
            01h separate status kept for each byte in receive buffer
        DX = port number
Return: AH = status
            00h successful
            3Ah invalid status storage specified
            41h no such port
            64h monitor mode already active
Note:   in monitor mode, MultiDOS redirects all BIOS video output to a serial
          port
SeeAlso: AH=25h