libc/opendir: Improve behaviour of union uniquifier:
libc/opendir: Improve behaviour of union uniquifier:

* Reopen the directory using openat(fd, ".", ...) instead of opening the
  pathname again. This fixes a race condition where the meaning of the
  pathname changes and allows a reopen with fdopendir().
* Always reopen the directory for union stacks, not only when DTF_REWIND
  is passed. Applications should be able to fchdir(dirfd(dir)) and
  *at(dirfd(dir), ...). DTF_REWIND now does nothing.

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