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.
My point was that the fact that if someone changes the mod time manually and it sets off the alert, that isn't necessarily a bad thing.

And if people aren't messing with random files, then sorting by {c,m}time is much more useful.