Fix MaxMind default data folders.

geoipupdate was recently split from geoip. It's still useful, and most users will want it, if only to download the initial databases. They both expected to use #{prefix}/share/GeoIP, which isn't possible, and having them use separate folders meant manually specifying the path everywhere.  I've modified them to share one folder, ${var}/GeoIP, instead. This plays nice with homebrew and allows the default settings to just work for both the update tool and the library.

libmaxminddb is the new data format library from MaxMind. As before, most users will want the database update tool that shipped with the older library, since it still works with the newer format databases.