feat: Improve statistics summary

feat: Improve statistics summary

The number of statistics counters has become very large, making the
output “ccache -s” hard to overview.

Improvements:

- The -s/--show-stats option now prints a more condensed overview where
  the counters representing “uncacheable calls” are summed as
  “uncacheable” and “errors” counters. Note: Scripts should use
  --print-stats instead of trying to parse the output of --show-stats.
- Added hit rate for direct/preprocessed hits/misses as well.
- Added a new -v/--verbose option, which makes --show-stats and
  --show-log-stats show more details.