Allow shutdown to unload modules that register bucket scheme's or codec's.

Allow shutdown to unload modules that register bucket scheme's or codec's.

* Change __ast_module_shutdown_ref to be NULL safe (11+).
* Allow modules that call ast_bucket_scheme_register or ast_codec_register
  to be unloaded during graceful shutdown only (13+ only).

ASTERISK-24796 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4428/
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Merged revisions 432058 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432059 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432060 65c4cc65-6c06-0410-ace0-fbb531ad65f3