INT 61 - PC/TCP kernel v2.05+ - "is_netnd" - CHECK IF NETWORK DESCRIPTOR VALID
        AH = 0Dh
        BX = possible network descriptor
Return: CF clear if valid
        CF set on error
            AX = error code (see INT 61"PC/TCP")
SeeAlso: INT 61"PC/TCP",INT 61/AH=00h"PC/TCP",INT 61/AH=08h"PC/TCP"
SeeAlso: INT 61/AH=22h

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