bug#19068: Mail file vars aren't derived from customized message-directory

[Eli Zaretskii]

> From: Kelly Dean <kelly@prtime.org>
> CC: 19068@debbugs.gnu.org
> Date: Fri, 30 Jan 2015 07:11:52 +0000
> 
> Eli Zaretskii wrote:
> >> Also,
> >> grep -r "~/Mail/" emacs-24.4/lisp/ | grep 'el:'
> >> gives 19 hits, all of which are inappropriate if ~/Mail isn't supposed to 
> >> be hardcoded. If ~/Mail is supposed to be hardcoded, then the 
> >> message-directory variable should be removed, to avoid misleading users.
> >
> > This is a red herring: all of these hits are either in comments or in
> > default values of other defcustoms.
> 
> If a user renames his ⌜Mail⌝ directory to ⌜mail⌝, then he'll want all the 
> things that previously used ⌜Mail⌝ to use ⌜mail⌝. Manually changing them all 
> is tedious and error-prone, so it'd be nice to have one place to make the 
> change. In Emacs, message-directory advertises itself as that place.

That's a separate issue.  I'm not at all sure the user will always
want to rename all of them, but we could offer an option to do that.