dhcp: systemd: support DHCPv6 prefix delegation
Add support to the internal DHCP client for requesting a prefix and distributing it to interfaces with 'shared' IPv6 mode.
The systemd-networkd API currently allows to request only a single prefix and so there will be issues when the number of downstream interfaces is greater than the number of /64 subnets available in the returned prefix; but this is still an improvement over the previous situation when no prefix was requested at all.