The previous implementation falsely reported all implicit capabilities
inherited from the authenticated user archetype. That caused a lot of
capabilities reported as missing, even if they were correctly granted.

This new implementation uses a different logic. Instead of seeking for
explicitly assigned capabilities, it searches for capabilities that are
not assigned to any of the user's role across the system.

Please refer to the inline documentation. This should be still used for
informative reports only, not for actual permissions evaluation. The
context has been ignored here, as well as all the overrides etc. This
patch just makes it a lesser evil.