machined: use different polkit actions for registering and creating a machine

machined: use different polkit actions for registering and creating a machine

The difference between these two operations are large: one is relatively
superficial: for "registration" all resources remain associated with the
invoking user, only the cgroup is reported to machined which then keeps
track of the machine, too. OTOH "creation" a scope is allocated in
system context, hence the invoked code will be owned by the system, and
its resource usage charged against the system.

Hence, use two distinct polkit actions for this, so that we can relax
access to registration, but keep access to creation tough.