Make greeter/autologin session explicitly request PAM conversation
    
    Now the greeter (and also the autologin code) has to say what
    PAM stack it wants the slave to run.  When that stack is ready,
    we emit the Ready signal as before, but now the Ready signal
    carries a string argument saying which service is ready to
    converse.
    
    When we support multiple PAM stacks, the greeter will call
    StartConversation for each stack, and will keep the UI
    associated with each stack disabled until the Ready signals
    come back one-by-one.