Properly clean up manager resources on exit

This patch does two things:
1) It properly unregisters the manager CLI commands
2) It cleans up AMI users on exit.  Prior to this patch, the AMI users
   were not being disposed of properly, resulting in a memory leak.

(closes issue ASTERISK-20646)
Reported by: Corey Farrell
patches:
  manager_shutdown.patch uploaded by Corey Farrell (license 5909)
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Merged revisions 375793 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375794 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375795 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375796 65c4cc65-6c06-0410-ace0-fbb531ad65f3