Introduce new Conversation object
    
    We want to eventually support having multiple
    simultaneous PAM conversations in one login
    screen (so, e.g., username/password, smart card, and
    fingerprint all work at the same time).
    
    This commit refactors the session code to be in terms
    of a conversation object.  With this change, it should
    be easier later to have multiple conversation objects.
    
    The conversation is named by the pam service the login
    screen is talking to.