Bugfix: Problems exposed by unit tests in f_conversion.

Bugfix: Problems exposed by unit tests in f_conversion.

The unit tests were disabled due to previous problems that are not no longer a problem.
This exposed a few areas that could be improved.

Rename "output" to "stream".
Make sure to call ferror_unlocked() to test for error.
Add loops to handle when fwrite_unlocked() doesn't write all the data but also doesn't throw an error.

Use "negative_or_zero" instead of "negative" to be more verbose and less confusing.