Fall back to registered country if necessary
When extracting geoip and geoip6 files from MaxMind's GeoLite2 Country database, we only look at country->iso_code which is the two-character ISO 3166-1 country code of the country where MaxMind believes the end user is located.
But if MaxMind thinks a range belongs to anonymous proxies, they don't put anything there. Hence, we omit those ranges and resolve them all to '??'. That's not what we want.
What we should do is first try country->iso_code, and if there's no such key, try registered_country->iso_code which is the country in which the ISP has registered the IP address.
In short: let's fill all A1 entries with what ARIN et. al think.
Found while updating Onionoo to use GeoLite2 instead of GeoLite. Sorry for not spotting this earlier.