Add option to specify allowed usage.
    
    Credentials can often be used both to accept and to initiate contexts.
    With this option admins can allow a specific usage only.
    This is to avoid allowing an unprivileged process to fool a remote
    client by allowing it to impersonate a server, when we only want to
    allow this service to use credentials to initiate contexts.
    
    Reviewed-by: Günther Deschner