Implements the following, as requested in `etc/TODO`:

** A function to check for customizable options that have been
   set but not saved, and ask the user whether to save them.
   This could go in kill-emacs-query-functions, to remind people
   to save their changes. If the user says yes, show them
   in a Custom buffer using customize-customized.

My copyright assignment info should be on file.

Cheers.

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