bug#583: Use XDG basedir spec for configuration files?

[Eli Zaretskii]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 31 Aug 2019 19:02:01 -0700
> Cc: 583@debbugs.gnu.org
> 
> Changing user-emacs-directory works coherently only if it's done very early 
> in 
> startup (which is all that Bug#15539 was asking for).

We should document what that "very early" means in practice.  Does it
mean in the early-init file? or can this be done reliably in the
normal init file as well?

The problem here is that some defcustom's refer to
user-emacs-directory, and so changing the value of
user-emacs-directory after those defcustoms were computed during
startup will leave those defcustoms pointing at the old place.