Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

- Use errx(3) instead of err(3) when checking if snprintf(3) succeeded.
snprintf(3) doesn't set errno in the tested cases.
- If the same argument reference (for example %1) was specified more than
once, the command didn't necessarily fit to the final command buffer. Fix
this using a dynamic sbuf buffer. Add a few regression tests for the case.

PR: bin/95079
No objections: freebsd-hackers