META: allow exits to restrict on something more sophisticated than IPs
Right now exit nodes only support IP-based restrictions... and in microdescriptors, those IP-based restrictions are summarized as port-based restrictions. From time to time people ask for restrictions based on other things, like AS ids, hostnames, etc.
I'm not sure I see a way to do this that would actually be worth the difficulty, much less one that would work... but perhaps there are benefits I'm not seeing?
Creating this as a meta-ticket so I can close other things as duplicates of it