Implement IPv6 first-hop gateways

[ipspace]

After #1785, it's possible to have subnets that do not use RA to generate default routes.

In those cases, we have to extend the "static route framework" has to IPv6, which also means we need first-hop gateways for IPv6. That includes default link gateway (first router attached to the link), anycast gateways, and VRRPv3 gateways.