openat: port lowest-fd to native MS-Windows Problem reported by Bruno Haible in: https://lists.gnu.org/r/bug-gnulib/2024-11/msg00081.html * lib/openat.c (openat_permissive): When save_cwd allocates an FD, allocate another one DFD and then close FD so that the later open returns FD (the lowest available fd), as POSIX requires.