Fix PD for DHCPv6

[sthaug at nethelp.no]

> > Additionally, host declarations for IPv6 don't support hardware
> > ethernet (as far as I'm aware), DHCPv6 doesn't use MAC addresses, you
> > need to find and use the DUID of the client, example:
> 
> Or use dhcpy6d which supports client MAC addresses. See
> https://dhcpy6d.ifw-dresden.de for details.

You still can't be sure it works, because the DUID doesn't necessarily
contain a MAC address.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no