logind: rework logic to decide whether lock + idle + display applies to a session

logind: rework logic to decide whether lock + idle + display applies to a session

Let's streamline the logic that decides whether the screen lock, idle
timeout or display election mechanism applies to a session class. Let's
add explicitly SESSION_CLASS_IS_XYZ() macros for each, and then resue
them at all suitable places, and refuse any attempts to use the
functionality on the wrong clases with a friendly error message.