b/f: remember passive_mode for keeping symmetric RTP in a session

If NAT flag (P-MsgFlags: 2) is passed only in the initial INVITE,
re-INVITE will set up the RTP stream to the private address, but not
enable symmetric RTP (comedia style send-to-where-i-get-rtp-from)
again. this fix remembers passive_mode for the call.

core/AmSession.cpp

@@ -79,6 +79,7 @@ AmSession::AmSession()
     m_dtmfDetector(this), m_dtmfEventQueue(&m_dtmfDetector),
     m_dtmfDetectionEnabled(true),
     accept_early_session(false),
+    passive_mode(false),
     rtp_interface(-1),
     refresh_method(REFRESH_UPDATE_FB_REINV),
     processing_status(SESSION_PROCESSING_EVENTS),
@@ -292,7 +293,6 @@ void AmSession::negotiate(const string& sdp_body,
     DBG("remote party doesn't support telephone events\n");
   }
 
-  bool passive_mode = false;
   if( sdp.remote_active || force_symmetric_rtp) {
     DBG("The other UA is NATed: switched to passive mode.\n");
     DBG("remote_active = %i; force_symmetric_rtp = %i\n",

core/AmSession.h

@@ -157,6 +157,9 @@ private:
 
   auto_ptr<AmRtpAudio> _rtp_str;
 
+  /** set up RTP stream as passive? (comedia style NAT aware) */
+  bool passive_mode;
+
   AmDynInvoke* user_timer_ref;
   
   void getUserTimerInstance();