sorcery, bucket: Change observer remove calls to take const callbacks struct.

* Make ast_sorcery_observer_remove() accept a const callbacks struct.

* Make ast_sorcery_observer_remove() tolerant of the sorcery parameter
being NULL.  Now it can be called within a module unload routine if the
sorcery initialization fails.

* Fix ast_sorcery_observer_add() to fail if the container link fails.
........

Merged revisions 403324 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403327 65c4cc65-6c06-0410-ace0-fbb531ad65f3