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.
It would break if someone opened a log in a text editor and *did* save (even if they made no changes).  In my opinion, that is much more likely than someone dropping random files into a log dir or randomly renaming files, as those are much harder to do accidentally.