systemd: Add plymouth-switch-root-initramfs.service to switch back to initramfs on...

systemd: Add plymouth-switch-root-initramfs.service to switch back to initramfs on shutdown

Add a plymouth-switch-root-initramfs.service, which will call
"plymouth update-root-fs --new-root-dir=/run/initramfs" to switch back
to the initramfs (when applicable).

Systemd will run this service before plymouthd receives the SIGTERM on
shutdown, so this will cause the plymouthd-fd-escrow helper to run
from the initramfs.

This avoids the plymouthd-fd-escrow helper keeping the rootfs busy.

Changes by Hans de Goede:
- Fix a couple of typos
- Add Conflicts=dracut-shutdown.service to plymouth-switch-root-initramfs.service
  dracut-shutdown.service restores the initramfs when it is _stopped_
  use Conflicts to make sure its ExecStop has run before we do
- Add a check for switching back to the initramfs to on_newroot () and dump
  the debug-buffer before the switch (while we still have access to /var/log).
- Also add plymouth-switch-root-initramfs.service to kexec.target.wants.
  kexec.target.wants uses --mode=shutdown, so the plymouthd-fd-escrow helper
  will run, so we need to switch to the initramfs.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>