netinet6: allow ND entries creation for all directly-reachable
destinations.

The 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-gatewaye routes 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.

Reviewed By: kbowling
Differential Revision: https://reviews.freebsd.org/D23695
MFC after: 1 month

(cherry picked from commit f998535a66b986f51dd65b5153d1a580d50ddfbe)