backend: hourly update without crontab or the like
Since I didn't want to bother with crontab, I've been using the attached patch for a while.
If no property is given the backend update will run one time.
When -Donionoo.cron.runonce=false is given the updater will
run after waiting a minute and then every hour after a successful
update run. (Thus, it might also be a solution for #13003 (moved).)
Please review.
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