This removes all references I could find to:
- credential provider
- authentication backend
- authentication provider
- auth provider
- auth backend
in favor of the unified:
- auth method
This commit splits ACL policies into more fine-grained capabilities.
This both drastically simplifies the checking code and makes it possible
to support needed workflows that are not possible with the previous
method. It is backwards compatible; policies containing a "policy"
string are simply converted to a set of capabilities matching previous
behavior.
Fixes#724 (and others).
