make Persistent=true smarter

this (partially) fix this self-reported issue: https://github.com/systemd-cron/systemd-cron/issues/14

I hope this python code is not too awfull

For anacrontab , **AccuracySec=** should maybe be replaced by **OnBootSec=** !?

AccuracySec would be usefull to randomize monthly/weekly/daily jobs.