bug#55579: 29.0.50; Abbrevs not saved on exit anymore

[Juri Linkov]

>> > Save abbrevs in ~/.emacs.d/abbrev_defs?
>>
>> BTW, I noticed that ~/.emacs.d/abbrev_defs
>> is visited without font-locking.  Here's the fix:
>
> Won't that make loading abbrevs slower and more expensive?  that file
> is normally not edited interactively, only if the user explicitly
> wants to do so.  So why is font-lock in it important?

This is part of recent efforts to add modes to .emacs.d files.
So far we added the mode tag to ~/.emacs.d/bookmarks, ~/.emacs.d/tramp,
~/.emacs.d/.emacs.desktop, ~/.emacs.d/network-security.data,
~/.emacs.d/places, ~/.emacs.d/projects.  But mode is still missing
in ~/.emacs.d/abbrev_defs and ~/.emacs.d/recentf.