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.