fixes for start-system-upgrade.service
Save upgrade systemd units to avoid confusion

After the switch-root back into initramfs, we don't have the fedup units
that the system has (upgrade-switch-root.service, upgrade-prep.service,
etc.).

When systemd tries to deserialize the old state, it can't find
the unit files for these units and just kind of... loses track of them.

Dracut used to solve this by saving its unit files into /run/systemd/system
before the switch-root - see this file from dracut 025:
  modules.d/98systemd/service-to-run.sh

So: similarly, we'll copy our upgrade unit files into the initramfs
before the switch-root so everything's findable.