Unsafe usage of readdir_r

When using `unix` on FreeBSD 15, we now get special warnings like these:

```
/usr/local/bin/ld: /home/ouranos/.ghcup/ghc/9.10.3/lib/ghc-9.10.3/lib/../lib/x86_64-freebsd-ghc-9.10.3-inplace/unix-2.8.7.0-inplace/libHSunix-2.8.7.0-inplace_p.a(HsUnix.p_o): in function `__hscore_readdir':
(.text+0x95): warning: warning: this program uses readdir_r(), which is unsafe.
/usr/local/bin/ld: /home/ouranos/.ghcup/ghc/9.10.3/lib/ghc-9.10.3/lib/../lib/x86_64-freebsd-ghc-9.10.3-inplace/unix-2.8.7.0-inplace/libHSunix-2.8.7.0-inplace_p.a(HsUnix.p_o): note: the message above does not take linker garbage collection into account
```

Not too sure if `unix` could put a warning on the function to propagate best practices, but I felt like you'd like to know. :)

Start by locating the unix source entry point that implements __hscore_readdir and uses readdir_r, then reproduce the linker warning on FreeBSD 15. Determine how the unix package should address or report the unsafe usage; done means the reported warning is resolved or the intended warning behavior is established without changing directory-reading behavior.

Written by the indexing model from the issue text.