- UID, CLASS and SCOPE are read at their first occurrence: logind writes
each once, and a later line saying otherwise is not the record's word.
- A comment says /run/systemd/sessions is systemd's private format, and
that a format change fails closed: no session, so nobody is the terminal.
- Tests that hold, each proved by the mutation it guards failing:
- a record naming UID=1001 for session-8.scope under user-1000.slice,
asked as uid 1000 (the record's UID check alone);
- session-a.b.scope with a passing record (the id's characters alone);
- session-9.scope/x.scope (the scope being the slice's direct child);
- a key said twice, either way round (first occurrence);
- the client sends its line only after the session was read, so reading
the session in the forward fails the test.