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

[Kelly Dean <kelly@prtime.org>]

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.

I lowercased my mail directory name both because it's easier to type that way (don't have to press shift), and because some other things already use the lowercase version by default, and there was no reason to use both upper- and lower-case versions.