Dynamic rss generation by index.pl for anonymous users

We currently only have the ability to dynamically generate rss for logged in users. This patch changes that if you set rss_allow_index >= 4 in vars. Settings of 3/2/1 already allowed logged-in-users/subscribers/admins, respectively. Set to 0 of course disables it.

When set it can be accessed as https://soylentnews.org/index.pl?content_type=rss. I'm not sure we'll ever use this but it could in the future allow for things like pulling a user supplied number of stories, pulling only stories from a certain category, etc... It's not as caching-friendly as the slashd service generated feeds but it's a damned sight more potentially versatile.