netinet6: simplify selectroute()

Effectively selectroute() addresses two different cases:
providing interface info for multicast destinations and providing
nexthop data for unicast ones. Current implementation intertwines
handling of both cases, especially in the error handling part.
Factor out all route lookup logic in a separate function,
lookup_route() to simplify the code.
Ensure consistent KPI: no error means *retifp is set and otherwise.

Differential Revision: https://reviews.freebsd.org/D35711
MFC after: 2 weeks