netinet: do route lookup when asked to join multicast group on ifindex 0

[glebius]

Summary

The code to do the route lookup was already there, but was used only for
the legacy IP_ADD_MEMBERSHIP when called without index. Do same lookup
for IP_ADD_MEMBERSHIP with index and what is more important for
MCAST_JOIN_GROUP, if the supplied index is 0. This is a neat feature and
Linux does that, so this should make a few applications easier portable to
FreeBSD.