- Use errx(3) instead of err(3) when checking if snprintf(3) succeeded.

- 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