geoip: add GeoIP2 and ASN support

* add GeoIP2 support
* load the ASN databases if present (v1 only, not available in v2, yet)

### Notes
* The ASN database is not yet available in the new GeoIP2 database format. 

### Todo
* ~~The ASN is available in the legacy format, but is not yet being utilized~~ Done.
* ~~It's two DBs, one for IPv4 and another for IPv6, so determine addr type first~~ Done.
but isn't that already done [here](https://github.com/smtpd/qpsmtpd/pull/139/files#diff-dd0bad9ed5baf80942b077c82f5396e4R205)?