Allow for possibility of index.pl feeds for anonymous users.

Currently index.pl can serve up dynamic rss/rdf/atom only to the admin's choice of admin, subscribers, or logged in users, this allows the site admin the choice of allowing anonymous users as well should they want to serve up dynamic feeds in addition to or in place of the static ones generated periodically by slashd.

I'm not saying we should necessarily do it but having the option would not be a bad thing.