Cleanup dnsmgr on exit.

Cleanup dnsmgr on exit.

* Cleanup dnsmgr thread and CLI commands on exit.

(issue ASTERISK-20649)
Reported by: Corey Farrell
Patches:
      dnsmgr-cleanup-1_8.patch (license #5909) patch uploaded by Corey Farrell
      dnsmgr-cleanup-10-11-trunk.patch (license #5909) patch uploaded by Corey Farrell
      Modified
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Merged revisions 377704 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 377705 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@377706 65c4cc65-6c06-0410-ace0-fbb531ad65f3