Fix PD for DHCPv6

[Steven Carr]

On 3 January 2018 at 13:05, alpha_one_x86
<alpha_one_x86 at first-world.info> wrote:
>     host laptop{
>         hardware ethernet b8:2a:72:9e:2a:26;
>         fixed-address6 2803:1920:0:1::2;
>     }

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:

host laptop {
            host-identifier option dhcp6.client-id
00:01:00:01:4a:1f:ba:e3:60:b9:1f:01:23:45;
            fixed-address6 2803:1920:0:1::2;
}