bug#16242: 24.3; wish: set init directory (.emacs.d) by commandline flag for easy custom environments

[Noam Postavsky]

>>> Does anybody object to adding an --init-dir switch?  It would just
>>> basically set `user-emacs-directory'.
>>
>> See https://debbugs.gnu.org/15539
>
> I see.  So the general consensus there seemed to be that this could add
> some possible maintenance complexity, and that there wasn't really a use
> case presented.
>
> This report does add a use case.

Furthermore, we're currently looking at setting user-emacs-directory
according to XDG in master ([1] or [2]), so I think we've accepted the
maintenance complexity this might bring anyway.

[1]: https://lists.gnu.org/archive/html/emacs-devel/2019-06/msg00154.html
[2]: https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00699.html