Bug#766902: @reboot jobs are run on package installation

[Alexandre Detiste]

> I had @reboot jobs existing, that had already run when my laptop booted.
> I then installed systemd-cron, and noticed this caused the jobs to run
> again.

Having "touch /run/systemd/<something>" in preinst script +
having the generator check for this file before generating @reboot jobs should do the trick ?