nbsed: fix building on operating systems that don't have errc(3)

One of the simplest ways of addressing this is to switch to errx(3). It
seems unclear what the absolute minimums are for C compilers required
for bootstrap, and EILSEQ wasn't codified until the 1994 Normative
Amendment 1 to C89, so its value (per NetBSD) is hard-coded here, too.

This introduces a small diff to upstream, but seems safer than altering
libnbcompat this close to a quarterly branch freeze. (There are also
some questions about libnbcompat's current state to be added to a PR.)

Relates to PR pkg/57970.

Tested on Fedora Linux 39, which was previously broken.

(gutteridge)