sip: Treat SIP 183 as progress too

It doesn't fix early media yet but brings us one step
closer to it:

   The 183 (Session Progress) response is used to convey information
   about the progress of the call that is not otherwise classified.  The
   Reason-Phrase, header fields, or message body MAY be used to convey
   more details about the call progress.

Change-Id: Ibf264f251e41c06a7b4839acc0d0853e6400291c

src/sip.c

@@ -156,7 +156,7 @@ void nua_callback(nua_event_t event, int status, char const *phrase, nua_t *nua,
 		if (leg->state == SIP_CC_INITIAL)
 			leg->state = SIP_CC_DLG_CNFD;
 
-		if (status == 180)
+		if (status == 180 || status == 183)
 			call_progress(leg, sip);
 		else if (status == 200)
 			call_connect(leg, sip);