INT 6B - NPC NCSI EXTENDED SERIAL I/O - SET/RETRIEVE VIRTUAL CIRCUIT CONFIG
        AH = 15h
        AL = virtual circuit number
        CL = direction (00h get, nonzero set)
        ES:BX -> buffer for/containing virtual circuit configuration
Return: ???
Note:   this function is also supported by TelAPI
SeeAlso: AH=13h,AH=14h

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