resolved: bind socket to interface during connect()

Apparently, IF_UNICAST_IF does not influence the routing decisions done
during connect(). But SO_BINDTODEVICE/SO_BINDTOINDEX does, which however
brings a lot of other semantics with it, we are not so interested in
(i.e. it doesn't not allow packets from any other iface to us, even if
routing otherwise allows it).

Hence, let's bind to the ifindex immediately before the connect() and
unbind right after again, so that we get the semantics we want, but not
the ones we don't.

Fixes: #11935
Replaces: #12004