better support for leap seconds

Various things in the code rely on leap seconds, mostly for GPS time conversion. Right now we're relying on tracking of leap seconds in the source, which is incredibly fragile and has bit us many times in the past. We need stop trying to track leap seconds and find a better way to get the needed info. For examples, coreutils tzdata tracks leap seconds in /usr/share/zoneinfo/leapseconds, which is what gpstime is now using.

This should be fixed before the next release so we can stop having to deal with this problem in the future.