The Austin Group is considering standardizing qsort_r(3) for the
upcoming revision of the standard. Unfortunately, Linux/glibc-based
systems provide a copy of this function that has a different prototype
than ours. Let's switch over to the same prototype that they are using.
There is no need to remain incompatible. Symbol versioning is used to
keep old binaries working.
My idea would be to commit this change not long after we unfreeze HEAD
after branching stable/12. In other words, this change will only become
part of FreeBSD 13.0.