The systemd DHCPv6 client requires a hardware address only to
determine the IAID; NM always overrides the IAID with its own and
therefore the hwaddr is not used.

Removing such requirement allows DHCPv6 to run over PPP, which is
useful with DHCPv6-PD to get a prefix from the ISP.

To test this, I set up a server with pppoe-server, radvd and the Wide
DHCPv6 server providing an address and a prefix. On the client, NM was
able to obtain a prefix using both dhcp=dhclient and dhcp=systemd.

Note that if there is no hardware address and you specify
ipv6.dhcp-duid=ll or ipv6.dhcp-iaid=mac, a warning will be emitted and
NM will use a random DUID/IAID.

#478
(cherry picked from commit 76a6a305)