Handle unresolvable routes (recursive routes that cannot be resolved) as
invalid, i.e. deprioritize tham and do not allow them for propagation.
Such route now shows as 'unresolvable' instead of 'unreachable'.
Add proper support for per-nexthop onlink flag in routes to handle next
hop addresses that are not covered by interface IP ranges. Supported by
kernel and static protocols.
Thanks to Vincent Bernat for the idea.