INT 61 u - PC/TCP kernel v2.05 - "net_listen" - ???
        AH = 23h
        BX = connection handle??? or FFFFh
        DX = subfunction???
        DS:SI -> ???
Return: CF clear if successful
            AX = ???
        CF set on error
            AX = error code (see INT 61"PC/TCP")
SeeAlso: INT 61/AH=00h"PC/TCP",INT 61/AH=13h