nspawn: complain and continue if machine has same id

nspawn: complain and continue if machine has same id

If --link-journal=host or --link-journal=guest is used, this totally
cannot work and we exit with an error. If however --link-journal=auto
or --link-journal=no is used, just display a warning.

Having the same machine id can happen if booting from the same
filesystem as the host. Since other things mostly function correctly,
let's allow that.

https://bugs.freedesktop.org/show_bug.cgi?id=68369