wc: Clean up and modernize.

• 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