Cleanup ast_run_atexits() atexits list.

* Convert atexits list to a mutex instead of a rd/wr lock.  The lock is
only write locked.

* Move CLI verbose Asterisk ending message to where AMI message is output
in really_quit() to avoid further surprises about using stuff already
shutdown.

(issue ASTERISK-20649)
Reported by: Corey Farrell
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Merged revisions 377165 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 377166 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 377167 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@377168 65c4cc65-6c06-0410-ace0-fbb531ad65f3