kamailio.cfg: few updates related to nat traversal

- added "co" flags to rtpproxy_manage() to change all IPs in sdp, many
  phone get confused if only media ip is changed
- add nat=yes parameter only for in branch route to avoid multiple
  occurences
(cherry picked from commit 0193489cca56f58b512f5379c078c98a366c17e2)

etc/kamailio.cfg

@@ -774,11 +774,13 @@ route[NATMANAGE] {
 	if (!(isflagset(FLT_NATS) || isbflagset(FLB_NATB)))
 		return;
 
-	rtpproxy_manage();
+	rtpproxy_manage("co");
 
 	if (is_request()) {
 		if (!has_totag()) {
-			add_rr_param(";nat=yes");
+			if(t_is_branch_route()) {
+				add_rr_param(";nat=yes");
+			}
 		}
 	}
 	if (is_reply()) {