[$] Namespaced file capabilities

The kernel's file capabilities mechanism is a bit of an awkward fit withuser namespaces, in that all namespaces have the same view of thecapabilities associated with a given executable file. There is a patch set under consideration that addsawareness of user namespaces to file capabilities, but it has brought forthsome disagreement on how such a mechanism should work. The question is, inbrief: how should a set of file capabilities be picked for any given usernamespace?