manager: be more accepting of callers with uids different from their session
    
    If a user runs su in their session, that switched user will now be
    running in a session that doesn't belong to it.  GDM won't allow a user
    access to the worker process associated with the session in this case.
    Instead, it will try to create a temporary just-in-time reauthentication
    channel so reauthentication can happen without having the user talking to
    another user's worker. Unfortunately, a logic error in the code means,
    the user won't access to its own just-in-time channel.
    
    This commit fixes that.