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

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.

Differential Revision: https://reviews.freebsd.org/D35710
MFC after: 2 weeks