Pass an Interactor to new_connection().

Thanks to the previous commit, this new parameter can replace two of
the existing ones: instead of passing a LogPolicy and a Seat, we now
pass just an Interactor, from which any proxy implementation can
extract the LogPolicy and the Seat anyway if they need it.

unix/sharing.c

@@ -297,7 +297,7 @@ int platform_ssh_share(const char *pi_name, Conf *conf,
     if (can_downstream) {
         retsock = new_connection(unix_sock_addr(sockname),
                                  "", 0, false, true, false, false,
-                                 downplug, conf, NULL, NULL);
+                                 downplug, conf, NULL);
         if (sk_socket_error(retsock) == NULL) {
             sfree(*logtext);
             *logtext = sockname;