httpcache.Cacher gains:
- Username/Password: Basic Auth, stripped before following redirects
- MaxAge: skip HTTP if local file mtime is within this duration
- MinInterval: skip HTTP if last Fetch attempt was within this duration
- Transform: post-process response body (e.g. extract .mmdb from tar.gz)
geoip.Downloader now builds an httpcache.Cacher via NewCacher(), removing
its own HTTP client. ExtractMMDB is now exported for use as a Transform.
check-ip-blacklist renamed to check-ip; adds -city-db / -asn-db flags
for GeoLite2 lookup (country, city, subdivision, ASN) printed after each
blocklist result.
