If a IPv6 router claims it's a valid router but doesn't have any public

If a IPv6 router claims it's a valid router but doesn't have any public
prefixes, don't install a default route for it.
If we did, some OS's will just stall trying to reach global addresses.
By not installing the default route, they can fall back to IPv4 faster.
This can be overridden with ipv6ra_always_default.