MINOR: listener: move the network namespace to the struct settings

The netns is common to all listeners/receivers and is used to bind the
listening socket so it must be in the receiver settings and not in the
listener. This removes some yet another set of unnecessary loops.
diff --git a/src/session.c b/src/session.c
index 52d3a1f..c303585 100644
--- a/src/session.c
+++ b/src/session.c
@@ -153,7 +153,7 @@
 	cli_conn->handle.fd = cfd;
 	*cli_conn->src = *addr;
 	cli_conn->flags |= CO_FL_ADDR_FROM_SET;
-	cli_conn->proxy_netns = l->netns;
+	cli_conn->proxy_netns = l->bind_conf->settings.netns;
 
 	conn_prepare(cli_conn, l->proto, l->bind_conf->xprt);
 	conn_ctrl_init(cli_conn);