logind: add function session_jobs_reply() to unify the create reply

The session_send_create_reply() function which notifies clients about
session creation is used for both session and user units. Unify the
shared code in a new function session_jobs_reply().

The session_save() will be called unconditionally on sessions since it
does not make sense to only call it if '!session->started', this will
also allow to update the session state as soon as possible.