webui: fix disabled state of service's PAC type
    
    Nested options (MS-PAC and PAD) of service's PAC type should be
    disabled if no value is supplied (default value is "Inherited
    from server configuration"). That was not the case - regression.
    
    This patch fixes it and along with it simplifies the update method
    of option_widget_base to be more comprehensible.
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>