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.