support systemd soft reboots

[cgwalters]

xref https://www.freedesktop.org/software/systemd/man/latest/systemd-soft-reboot.service.html

We could theoretically do this by default if we detect there's no kernel changes...but per the doc it will cause state leakage in corner cases around things like kernel sysctl.d, module loading blacklists, etc.

systemd (understandably) just made this an opt-in thing. I think to start, we should do the same...just add the basic enablement features of setting up the next root.

Then users that want could invoke e.g.:

ostree prepare-next-root

And hmm... we should have a flag that detects the case where the kernel is skewed, and errors out by default. But in theory we should also allow this, but ensure we've e.g. bind mounted in the old kernel data.