CEL: Protect data structures during reload and shutdown.

The CEL data structures need to be protected during a configuration reload
and shutdown.  Asterisk crashed during a shutdown because CEL events were
still in flight and the CEL data structures were already destroyed.

* Protected the appset and linkedids ao2 containers using the reload_lock.

* Added NULL checks before use of the appset and linkedids ao2 containers
in case the CEL module is already shutdown.

* Fixed overloading of the linkedids held objects reference count.  During
shutdown any held objects would be leaked.

* Fixed memory leak of linkedids held objects if the LINKEDID_END is not
being tracked.  The objects in the linkedids container were not removed if
the LINKEDID_END event is not used.

* Added access protection to the appset container during the CLI "cel show
status" command.

* Made CEL config reload not set defaults if the cel.conf file is invalid.

(closes issue AST-1253)
Reported by: Guenther Kelleter

Review: https://reviewboard.asterisk.org/r/3127/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@406417 f38db490-d61c-443f-a65b-d21fe96a405b