Current assumption is that kernel-handled rtadv prefixes along with
the interface address prefixes are the only prefixes considered in
the ND neighbor eligibility code.
Change this by allowing any non-gatewayed to be eligible.
This will allow DHCPv6-controlled routes to be correctly handled by
the ND code.
Refactor nd6_is_new_addr_neighbor() to enable more deterministic
performance in "found" case and remove non-needed
V_rt_add_addr_allfibs handling logic.
This is the alternative form of D15404 && D15405, not involving new route flags.