sys: `rb_add_elements()` don't overwrite existing content

`rb_add_elements()` is undocumented and currently unused. I'd like to use it. My problem is that the function overwrites existing content. Hence it doesn't return how many elements were added.
The corresponding get function `rb_get_elements()` only reads as many elements as there are stored, and returns the number of actually read elements.

Are there objections to change the behavior of `rb_add_elements()` not to overwrite existing elements?