tarsnap-archive - added use of config file

This removes my hardcoded backup paths, while letting me keep labels.
Shifting lots over to cron

Specifically, working out the "daily/weekly/monthly" label for the
backup felt like it was reinventing the wheel. It's simpler now.
prune uses the backup list now

That list is at /etc/tarsnap-cron.conf now.
Fixed typos.
Made prune actually do its thing instead of print it for inspection.
Crontab example: let's put the binaries in /usr/local/bin