Report the right address in connection setup errors.

backend_socket_log was generating the IP address in its error messages
by means of calling sk_getaddr(). But sk_getaddr only gets a SockAddr,
which may contain a whole list of candidate addresses; it doesn't also
get the information stored in the 'step' field of the Socket that was
actually trying to make the connection, which says _which_ of those
addresses we were in the middle of trying to connect to.

So now we construct a temporary SockAddr that points at the
appropriate one of the addresses, and use that for calls to plug_log
during connection setup.