Improve log summary nagios check significantly

Fixes issue https://github.com/osuosl-cookbooks/rdiff-backup/issues/7

- Check now uses the two most recent job logs instead of just the current one.
- Check now only uses the most recent job run for each job.
- Check now lists failed and missing jobs.
- Check now correctly lists number of jobs, even when jobs are missing or have been run multiple times per day.
- Check now ignores removed jobs.
- Check now notifies of missing or failed purges.
Alphanumeric sort would stop working if someone ever changes the names, if someone accidentally dumps some files in the dir, if an upgrade makes the default logs change, etc.