Some tweaks to statistics.

Changes to directory request statistics:

- Rename GEOIP statistics to DIRREQ statistics, because they now include
  more than only GeoIP-based statistics, whereas other statistics are
  GeoIP-dependent, too.
- Rename output file from geoip-stats to dirreq-stats.
- Add new config option DirReqStatistics that is required to measure
  directory request statistics.
- Clean up ChangeLog.

Also ensure that entry guards statistics have access to a local GeoIP
database.