logind: rework the special casing we give root's sessions

logind: rework the special casing we give root's sessions

Let's add an explicit session class "user-early" for this, so that
change of behaviour on logind is primarily bound to the "class"
property, and not some explicit root checks. This has the benefit that
we can be more fine grained with implying this class: only do so for tty
sessions, not others.