Provide custom-prompt-customize-unsaved-options.

* doc/emacs/custom.texi (Saving Customizations): Mention
`custom-prompt-customize-unsaved-options'.

* etc/NEWS: Mention `custom-prompt-customize-unsaved-options'.

* etc/TODO: Remove its entry.

* lisp/cus-edit.el (custom-prompt-customize-unsaved-options): Add a
mechanism for prompting user about unsaved customizations.
(Bug#19328)