The decc$feature_set_value function sets the default value or
    the current value (as determined by the mode argument) for the
    feature specified by index.

    If this function is successful, it returns the previous value.

    On error, -1 is returned and errno is set to indicate the error.

    See also decc$feature_get, decc$feature_get_index, decc$feature_
    get_name, decc$feature_get_value, decc$feature_set, decc$feature_
    show, and decc$feature_show_all.