nsresourced: check polkit before executing our operations

nsresourced: check polkit before executing our operations

Let's tighten rules on namespace operations: let's always ask PK for
permission before doing anything.

Note that if polkit is absent we'll still allow things, and the default
PK policy will also still allow things, but there's now a clear way how
people can not allow things if they want, by modifying the PK policy.