res_pjsip_messaging: send message to a default outbound endpoint

In some cases messages need to be sent to a direct URI (sip:<ip address>). This
patch adds in that support by using a default outbound endpoint.  When sending
messages, if no endpoint can be found then the default one is used.

To facilitate this a new default_outbound_endpoint option was added to the
globals section for pjsip.conf.

Review: https://reviewboard.asterisk.org/r/2944/
........

Merged revisions 403680 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403687 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip/include/res_pjsip_private.h

@@ -105,4 +105,15 @@ int ast_sip_for_each_channel_snapshot(const struct ast_endpoint_snapshot *endpoi
 				      on_channel_snapshot_t on_channel_snapshot,
 				      void *arg);
 
+/*!
+ * \brief Retrieve the name of the default outbound endpoint.
+ *
+ * \note This returns a memory allocated copy of the name that
+ *       needs to be freed by the caller.
+ *
+ * \retval The name of the default outbound endpoint.
+ * \retval NULL if configuration not found.
+ */
+char *ast_sip_global_default_outbound_endpoint(void);
+
 #endif /* RES_PJSIP_PRIVATE_H_ */