There seems to be no -nocreate parsing in main().

Why do you need this child? Could you fchdir() in the runat process directly, and then do system(), without forking?

This should be exit(WEXITSTATUS(sysret)); at least. Better, it can be

		if (WIFSIGNALED(sysret))
			raise(WTERMSIG(sysret));
		if (WIFEXITED(sysret))
			exit(WEXITSTATUS(sysret));

If you want to propagate the status of child to invoking shell, see above.

In fact, why do you need system(3)? This kind of utilities, I believe, set up the exec(3) args vector by chomping consumed arguments, prepend "sh", "-c", and call exec(3) function. Then you do not need to try to perfectly pass the child exit status.

system(3) does a lot of signal blocking during
vfork(), etc. If you don't think that's necessary and
it's just exec(), then I an do that?

Yes, I suggest to use exec(). We have nothing to do after the child is spawn. Catching the child status and trying to propagate it causes the complications you already noted.

BTW, if you open file and directory with O_CLOEXEC, you do not even need to close them manually.

If you want you can use the SPDX short form. It's an iso standard and project's template in current style(9) or here: https://docs.freebsd.org/en/articles/license-guide/

I think function arguments aren't for utilities.

Since we have a manual for that it might be helpful to link it here. It will also be clickable in "modern" manual readers.

Since you are using exec now, it might be better to not concatenate all args into one string. Instead, allocate char *[] array and assign *argv[] elements to it.

The advantage is that the original quoting of the shell is preserved.

This open needs O_CLOEXEC too.

The "-c" argument for /bin/sh requires that the
entire command be in a single string argument.

style(9) asks for a blank line before and after this, and sometimes this is controversial to clean up later. I also don't think that the copyright line has a period. Sorry to be pedantic, licenses are very specific and a bit scary.

Why not markup examples section?

The "-" was for a project Warner started, never went anywhere, and abandoned. It is still recommended in a style guide anywhere? We tried to clean them all up, but there's a lot of doc proliferation.

Not sure what you mean here, but I made "file"
a .Ar.