Update: Improve Basic List content read return handling when at end range or buffer.

Update: Improve Basic List content read return handling when at end range or buffer.

The return state returns `F_okay_eos` or `F_okay_stop` accordingly.
This can be inferred to be content found, but not consistently.
It is better to return `F_content_found`.
Anything using this can now more consistently rely on the `F_content_found` status.
The end of string and buffer location checks should already be being performed by the caller anyway.

The **API** doesn't go into details on this behavior which leaves room for this change without breaking **API** or **ABI**.

This is back ported from ongoing changed from the `0.8` branch.