netinet6: factor out cached route lookups from selectroute().

[melifaro]

Summary

Currently selectroute() contains two nearly-identical versions of
the route lookup logic - one for original destination and another
for the case when IPV6_NEXTHOP option was set on the socket.

Factor out handling these route lookups in a separation function to
improve readability.
This change also fixes handling of link-local IPV6_NEXTHOPs.
While here, require all selectroute() consumers to pass both retnh and retifp to simplify return logic.

Test Plan

16:13 [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 [0.392s]
test_ip6_output.py:TestIP6Output::test_output6_nhop -> passed [0.436s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[empty] -> passed [0.399s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[ifsame] -> passed [0.407s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[ipandif] -> passed [0.502s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[iponly1] -> passed [0.401s]
test_ip6_output.py:TestIP6Output::test_output6_pktinfo[nolocalip] -> passed [0.416s]
test_ip6_output.py:TestIP6OutputLL::test_output6_linklocal -> passed [0.488s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff02] -> passed [0.420s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff05] -> passed [0.422s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff08] -> passed [0.413s]
test_ip6_output.py:TestIP6OutputMulticast::test_output6_multicast[ff0e] -> passed [0.403s]
test_ip6_output.py:TestIP6OutputNhopLL::test_output6_nhop_linklocal -> passed [0.424s]

[melifaro]

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