Added option for automatic reboot after a kernel upgrade.

Created by: phdoerfler

When meltdown and spectre happened I wrote a little script to automatically restart my mail server after a (then almost daily) kernel upgrade. This is a cleaned up version. I might put this on nixpkgs eventually but for now I wanted to have it out there at least for NMS so others can sleep better at night knowing their server will apply a kernel upgrade by itself.