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.
I'm curious as to why you mention this, since I don't see why the alphanumeric sort wouldn't always work.  Additionally, sorting by modification time would make it slightly harder to test because you would also need to manually change the file modification times if you are editing the logs to see how it handles different situations.