Add handling for the special group system:masters

[raesene]

What would you like to be added

The system:masters group in Kubernetes is special in that it always has full permissions to a cluster, even if there are no clusterrolebindings for it. There's a reference in the API server source code here

At the moment kubectl-who-can doesn't account for that, so will incorrectly not report members of system:masters as having all rights, if there are no clusterrolebindings mentioning them.

The correct solution is likely to just report true where the principal is an authenticated member of that group.

Why is this needed

Without this, incorrect conclusions might be reached about who has access to what :)