bridges/bridge_native_rtp: Reconfigure bridge on removal of framehook

This patch is a re-do of r414122.

When r414122 was merged, a major problem with it was uncovered. UNBRIDGE soft
hangup flags have a catastrophic effect on the pbx core if they leak out from
the bridge layer: the channel gets hung up. With the number of threads
involved in a blind transfer, and with the initial patch, it was likely that
this would occur. This caused a large number of test failures

This patch is nearly identical with the one proposed in r414122, save for the
following changes:
 - We explicitly clear the UNBRIDGE flag when setting an after goto on a
   channel in a bridge
 - Defensively, if we encounter an UNBRIDGE flag in the pbx core, we handle it

https://reviewboard.asterisk.org/r/3585/
........

Merged revisions 415443 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@415444 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/channel.h

@@ -4272,6 +4272,22 @@ struct ast_bridge *ast_channel_get_bridge(const struct ast_channel *chan);
  */
 int ast_channel_is_bridged(const struct ast_channel *chan);
 
+/*!
+ * \brief Determine if a channel is leaving a bridge, but \em not hung up
+ * \since 12.4.0
+ *
+ * \param chan The channel to test
+ *
+ * \note If a channel is hung up, it is implicitly leaving any bridge it
+ * may be in. This function is used to test if a channel is leaving a bridge
+ * but may survive the experience, if it has a place to go to (dialplan or
+ * otherwise)
+ *
+ * \retval 0 The channel is not leaving the bridge or is hung up
+ * \retval non-zero The channel is leaving the bridge
+ */
+int ast_channel_is_leaving_bridge(struct ast_channel *chan);
+
 /*!
  * \brief Get the channel's bridge peer only if the bridge is two-party.
  * \since 12.0.0