Click-ify various listnames to point to
http://lists.freebsd.org/mailman/listinfo/. While I'm here, remove the
digest email lists since that's an account preference under Mailman. The
freebsd-install list doesn't exist any more (now is -config), and very
hastily thump out documentation on how to subscribe/unsubscribe to/from
various mailing lists (though this should be much more obvious and easier
for users since every email has a URL to click on at the bottom of each
message).