Add 'GETINFO ip-to-country/available'

Hi Nick. Very minor ask but if we had a 'GETINFO ip-to-country/available' option to determine if tor has a geoip database available that would simplify Stem a bit. Stem tracks 'is the geoip database available' so it can avoid 'GETINFO ip-to-country/*' requests that are doomed to fail anyway.

Thanks!

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