logind: when registering a new session always use previous session info from cgroup path rather than audit

Previously for cases like "su" or "sudo" where a session is attempted to
be created from within an existing one we used the audit session ID to
detect this and in such a case we simple returned the session data of
the original session a second time.

With this change we will now use the cgroup path of the calling path to
determine the old session, i.e. we only rely on our own session
identification scheme, instead of audits.

We will continue to keep the audit session ID and ours in sync however,
to avoid unnecessary confusion.