The decc$feature_get_value function retrieves a value for the
    feature specified by index. The mode determines which value is
    returned.

    The default value is what is used if not set by a logical name or
    overridden by a call to decc$feature_set_value.

    If mode = 4, then the initialization state is returned. Values
    for the initialization state are:

       0 not initialized
       1 set by logical name
       2 forced by decc$feature_set_value
       -1-initialized to default 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_set, decc$feature_set_value, decc$feature_
    show, and decc$feature_show_all.