nspawn: register containers with both the system-wide and the per-user machined instance

nspawn: register containers with both the system-wide and the per-user machined instance

Let's make sure each container run by unpriv users is known by both
instances. Making it known by the host instance ensures it is resovlable
by nss-myhostname (and various other tools), but making it known by the
per-user instance means the user can fully manage it on their own.

This simplifies the code a bit too, i.e. we always allocate a scope
ourselves now, and just register it, instead of letting machined do that
under some circumstances. THere's little value in that and it simplifies
things for us quite a bit.