netinet6: simplify selectroute()

[melifaro]

Summary

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

Test Plan

17:54 [0] m@devel2 s kyua test -k /usr/tests/sys/netinet6/Kyuafile test_ip6_output.py
test_ip6_output.py:TestIP6Output::test_output6_base -> passed [0.413s]
test_ip6_output.py:TestIP6Output::test_output6_nhop -> passed [0.393s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[empty] -> passed [0.393s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[ifsame] -> passed [0.404s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[ipandif] -> passed [0.419s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[iponly1] -> passed [0.411s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[nolocalip] -> passed [0.419s]
test_ip6_output.py:TestIP6OutputLL::test_output6_linklocal -> passed [0.412s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff02] -> passed [0.399s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff05] -> passed [0.408s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff08] -> passed [0.392s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff0e] -> passed [0.401s]
test_ip6_output.py:TestIP6OutputNhopLL::test_output6_nhop_linklocal -> passed [0.445s]