ARI/PJSIP: Add the ability to redirect (transfer) a channel in a Stasis app

This patch adds a new feature to ARI to redirect a channel to another server,
and fixes a few bugs in PJSIP's handling of the Transfer dialplan
application/ARI redirect capability.

*New Feature*
A new operation has been added to the ARI channels resource, redirect. With
this, a channel in a Stasis application can be redirected to another endpoint
of the same underlying channel technology.

*Bug fixes*
In the process of writing this new feature, two bugs were fixed in the PJSIP
stack:
(1) The existing .transfer channel callback had the limitation that it could
    only transfer channels to a SIP URI, i.e., you had to pass
    'PJSIP/sip:foo@my_provider.com' to the dialplan application. While this is
    still supported, it is somewhat unintuitive - particularly in a world full
    of endpoints. As such, we now also support specifying the PJSIP endpoint to
    transfer to.
(2) res_pjsip_multihomed was, unfortunately, trying to 'help' a 302 redirect by
    updating its Contact header. Alas, that resulted in the forwarding
    destination set by the dialplan application/ARI resource/whatever being
    rewritten with very incorrect information. Hence, we now don't bother
    updating an outgoing response if it is a 302. Since this took a looong time
    to find, some additional debug statements have been added to those modules
    that update the Contact headers.

Review: https://reviewboard.asterisk.org/r/4316/

ASTERISK-24015 #close
Reported by: Private Name

ASTERISK-24703 #close
Reported by: Matt Jordan
........

Merged revisions 431717 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431718 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stasis_app.h

@@ -485,6 +485,17 @@ void stasis_app_control_clear_roles(struct stasis_app_control *control);
  */
 int stasis_app_control_continue(struct stasis_app_control *control, const char *context, const char *extension, int priority);
 
+/*!
+ * \brief Redirect a channel in \c res_stasis to a particular endpoint
+ *
+ * \param control Control for \c res_stasis
+ * \param endpoint The endpoint transfer string where the channel should be sent to
+ *
+ * \return 0 for success
+ * \return -1 for error
+ */
+int stasis_app_control_redirect(struct stasis_app_control *control, const char *endpoint);
+
 /*!
  * \brief Indicate ringing to the channel associated with this control.
  *