build: mention ftp redirector in release announcements Now that http://ftpmirror.gnu.org/PACKAGE/ redirects to a nearby mirror, we should promote its use in release announcements. Meanwhile, mention the mirror list. * top/maint.mk (gnu_rel_host, url_dir_list): Provide defaults for values that used to come from cfg.mk; mention FTP redirect URL. * build-aux/announce-gen: Mention the mirror list. Suggested by Karl Berry. Signed-off-by: Eric Blake <ebb9@byu.net>