systemctl: if kexec fails with --force, continue to reboot normally

systemctl: if kexec fails with --force, continue to reboot normally

When we are in late shutdown, and for whatever reason kexec fails, we should
proceed with a normal reboot. Network is down and sessions have been terminated
when we attempt to do the kexec, so rebooting normally is a better solution.

Logs from the case where the kexec kernel is not usable:
Mar 08 11:23:10 fuefi systemd[1]: Reached target Final Step.
Mar 08 11:23:10 fuefi systemd[1]: Starting Reboot via kexec...
Mar 08 11:23:10 fuefi systemctl[1480]: Cannot find the ESP partition mount point.
Mar 08 11:23:10 fuefi systemctl[1480]: Failed to load kexec kernel, continuing without.
Mar 08 11:23:10 fuefi systemd[1]: Shutting down.
... and then we proceed to do a normal reboot

Related to #7730.