access: rework struct credentials API

Struct credentials is a cache of user's universal privileges. It
is static and is never changed after creation. That is a problem.
If a user privileges are updated, it is not reflected in his
existing credentials caches.

This patch reworks credentials API so as now this struct is not
just a container for several numbers. It is an object with
standard methods like create(), destroy(). A credentials object
still is not updated together with its source user, but now at
least the API allows to fix that.

Next patch will link all struct credentials of a user into a list
via which the user will be able to keep the credentials up to
date.

Part of #2763

(cherry picked from commit a8c3ebdbfc97b72832ebc5d87b681a310cce9589)
(cherry picked from commit 6b15dce614cfc3b14a12b66819737263a5089eaf)