logind: fix serialization/deserialization of user's "display session"

logind: fix serialization/deserialization of user's "display session"

Previously this was serialized as part of the user object. This didn't
work however, as we load users first, and sessions seconds and hence
referencing a session from the user load logic cannot work.

Fix this by storing an IS_DISPLAY property along with each session, and
make the session with this set display session when it is loaded.