[FEATURE] Scheduler: Allow execution using "at"-daemon

Add possibility to execute scheduler via "at"-daemon instead of via cron.
This allows the scheduler more flexibility when to be invoked next.

This can be useful in environments where no cronjob can be set easily,
but access to at daemon is possible.

Change-Id: Ib450e59d76e23eb2eeb1ab4769f49d8ba8bd96b5
Resolves: #34227
Releases: 6.0
Reviewed-on: http://review.typo3.org/9177
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn