* Add a test that causes mbrtowc() to return 0.
* Add a test that causes mbrtowc() to return -2.
Sponsored by: Klara, Inc.
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38605
* Drop <err.h>, which is unnecessary since we use libxo.
* As per POSIX, report an error if output fails.
* Fix some type mismatches.
* Use bool instead of int where appropriate.
* Avoid repeatedly checking for a null filename.
* Miscellaneous other tidying.
* Add tests (partly derived from work performed by SHENG-YI HONG <i19780219111@kimo.com>).
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38496
