Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Migrate most of tools/regression/usr.bin/ to the new tests layout.

I'm starting with the easy cases. The leftovers need to be looked at a
bit more closely.

Note that this change _does_ modify the code of the old tests. This is
required in order to allow the code to locate the data files in the
source directory instead of the current directory, because Kyua
automatically changes the latter to a temporary directory.

Also note that at least one test is known to be broken here. Actually,
the test is not really broken: it's marked as a TODO but unfortunately
Kyua's TAP parser currently does not understand that. Will have to be
fixed separately.

- 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