Fix DTMF atxfer running h exten after the wrong bridge ends.

When party B does an attended transfer of party A to party C, the
attending bridge between party B and C should not be running an h exten
when the bridge ends.  Running an h exten now sets a softhangup flag to
ensure that an AGI will run in dead AGI mode.

* Set the AST_FLAG_BRIDGE_HANGUP_DONT on the party B channel for the
attending bridge between party B and C.

(closes issue AST-870)

(closes issue ASTERISK-19717)
Reported by: Mario

(closes issue ASTERISK-19633)
Reported by: Andrey Solovyev
Patches:
      jira_asterisk_19633_v1.8.patch (license #5621) patch uploaded by rmudgett
Tested by: rmudgett, Andrey Solovyev, Mario
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Merged revisions 364060 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 364065 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@364082 65c4cc65-6c06-0410-ace0-fbb531ad65f3