bug#19328: [PATCH] Add mechanism to prompt about unsaved customizations

[Cameron Desautels <camdez@gmail.com>]

> FWIW, These are the changes I made to `customize-unsaved':

> ;; 1. By default, ignores preferences in `customize-customized-ignore'.
> ;; 2. Added prefix arg to override `customize-customized-ignore'.
> ;; 3. When not interactive and there are changes, ask for confirmation.
> ;; 4. Always returns `t', so it can be used as a
> ;;    `kill-emacs-query-functions' hook.
> ;; 5. Wrap in `condition-case' and reissue error.

Thank you for sharing that!  I looked through it to see if I'm missing
anything important.  Your code is much more ambitious than the humble
patch I'm offering.  I definitely see the rationale behind your work
on `cus-edit+.el' but I think the important parts of it are orthogonal
to what I've done here.

WRT the five items above, 1 & 2 can be disregarded here because
`customize-customized-ignore' only exists in your library (and is only
necessary because of the elimination of the "changed outside of
Customize" state).

3-5 I cover differently--a new function rather than changing the
behavior of `customize-unsaved'--and this way has the benefit of being
minimally invasive (i.e. shouldn't break anyone else's code).

TLDR I still think it makes sense to merge the patch.

-- 
Cameron Desautels <camdez@gmail.com>