Prefer ~/.config/emacs to ~/.emacs.d if neither exists

Prefer ~/.config/emacs to ~/.emacs.d if neither exists

That way, when Emacs starts in a fresh home directory,
it prefers the new (XDG) convention rather than the old one.
* lisp/files.el (locate-user-emacs-file): Make the parent
directories of user-emacs-directory if needed.  This is useful
if user-emacs-directory is "~/.config/emacs" and "~/.config"
does not yet exist.
* lisp/startup.el (command-line):
* lisp/subr.el (user-emacs-directory):
Prefer XDG_CONFIG_HOME to ~/.emacs.d if neither exists.