machined: Skip root user namespace check for user managers

machined: Skip root user namespace check for user managers

You can register whatever process you want in the user machined instance
that is running in the same namespace as pid 1 as machined won't be allowed
to do anything privileged anyway that could be dangerous when running as a user
instance.

We have to skip the check as we user machined instances don't have
privileges to inspect pid1's user namespaces.