Clone dirhandles without fchdir

[Leont]

This is a WIP for fixing #23010 based on @ilmari's suggestion. There currently are two problems with it.

1. It doesn't have a Configure check yet for fdopendir
2. t/op/threads-dirh.t fails because it's assuming that the dirhandles in different threads are independent. Given that the same isn't true of filehandles, I think this is the wrong thing to aim at anyway.

[tonycoz]

I think it's one reasonable fix, though I think it would be an entry in incompatible changes in perldelta.

I'm not sure it's safe for multiple threads to readdir() from the same underlying fd: It's UB for both parent and child of a fork() to read from their corresponding copies of the DIR, which is the closest case I can think of to this, so I'm not sure this will be thread safe.

From my testing with simple test code, glibc gets confused about the telldir() position for the new handle.

One option might be to openat(orig_dir_fd, ".", O_DIRECTORY | ...) which, assuming no permission change on ., should give us an independent handle, which can be positioned like we do in the current code.

Of course, the simplest option is to just not pass the DIR * into the new interpreter, which resolves the original issue in #10387, but is as backward (in)compatible as this change.