XDG Compliant user (early) packages tree.

XDG Compliant user (early) packages tree.

With this changeset, the location for a user's local packages, the ones
that make up `early-packages', is now under ${XDG_DATA_HOME}/sxemacs.
That is normally ~/.local/share/sxemacs, which is the directory used when
the XDG environment variable isn't set.

If the user has forced use of the old legacy ~/.sxemacs for their init
directory, then that is where the early-packages will be instead of the
XDG location.  A warning is displayed if package subdirectories are
detected in any 2 or more of ~/.sxemacs, ${XDG_CONFIG_HOME}/sxemacs, and
${XDG_DATA_HOME}/sxemacs (symlinked directories are ignored).

There is also a new command line switch, `-user-pkgs-directory' to set the
early-packages directory to any location.

	* lisp/package-admin.el (package-admin-find-top-directory): Use
	`user-packages-topdir' instead of `user-init-directory'.
	(package-admin-get-install-dir): Ditto.

	* lisp/package-get.el (package-get-package-index-file-location):
	Default location is now in `user-packages-topdir' instead of
	`user-init-directory'.
	(package-get-install-to-user-directory): Renamed from
	`package-get-install-to-user-init-directory'.  The latter is
	defvaralias'd back.
	(package-get-maybe-save-index): Use `user-packages-topdir' instead
	of `user-init-directory'.

	* lisp/packages.el (packages-find-user-topdir): New.  Returns
	`user-packages-topdir'.
	(packages-compute-package-locations): Rename the arg to
	`user-packages-topdir', and update the doc string.

	* lisp/startup.el (command-line-do-help): Add `-user-pkgs-directory'.
	(command-line-early): For `-user' set an XDG compliant directory
	for `user-packages-topdir' if possible, fall back to the legacy
	.sxemacs otherwise.
	(startup-setup-paths): Use #'packages-find-user-topdir to set
	`user-packages-topdir'.

	* src/emacs.c (vars_of_emacs): New var, `Vuser_packages_topdir'.
	(main_1): Handle new command line arg, `-user-pkgs-directory'. Set
	`Vuser_packages_topdir'.

Signed-off-by: Steve Youngs <steve@sxemacs.org>