paths: remove DEV_NULL
    
    The platform-specific path to /dev/null is provided by the Python standard
    library in os.devnull.
    
    Replace all uses of paths.DEV_NULL with os.devnull and remove DEV_NULL.
    
    https://fedorahosted.org/freeipa/ticket/6474
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>