Use XDG conventions more consistently

Use XDG conventions more consistently

Fit in better with the XDG conventions.
Something like this was suggested in 2008 (Bug#583)
and the XDG conventions seem to have settled down by now.
* doc/emacs/custom.texi (Init File, Init Syntax, Find Init):
* doc/lispref/files.texi (Standard File Names):
* doc/lispref/os.texi (Init File):
* doc/misc/url.texi (Customization):
* etc/NEWS:
Adjust accordingly.
* lisp/startup.el (startup--load-user-init-file):
If init-file-name is nil, do not load from it; instead
just use the alt-file.
(find-init-path): Remove; no longer used.
(command-line): Don't check twice for XDG.
Look at XDG_CONFIG_HOME instead of assuming it's ~/.config.
Prefer XDG configuration if it exists; the user can disable
this by setting XDG_CONFIG_HOME to some other place.
* lisp/subr.el (user-emacs-directory):
Prefer XDG configuration if it exists.