nspawn: restart the whole systemd-nspawn@.service unit on container reboot (#4613)

nspawn: restart the whole systemd-nspawn@.service unit on container reboot (#4613)

Since 133 is now used in a few places, add a #define for it.
Also make the status message a bit informative.

Another issue introduced in b006762. The logic was borked, we were supposed
to return 0 to break the loop, and 133 to restart the container, not the other
way around.

But this doesn't seem to work, reboot fails with:
Nov 08 00:41:32 laptop systemd-nspawn[26564]: Failed to register machine: Machine 'fedora-rawhide' already exists
So actually the version before this patch worked better, since 133 > 0 and we'd
at least loop internally.