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
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.