Instead, check the number of rows above the cursor position,
and error if passed a too large number.

… too large argument.

The implementation is a proof-of-concept, and not fully polished yet.

- Also parse multi-digit cursor positions correctly.
- Clear the terminal after sending the code.
- accept an u16, as terminal can be up to 65536 chars wide or long
- just read a limited buffer, not an entire line;
I can use a fixed-size buffer because of the above.
- note two current FIXMEs in the code.
- restrict this code to UNIX systems for now.