osutil: fix potential wrong fd close According to POSIX closedir [1]: If a file descriptor is used to implement type DIR, that file descriptor shall be closed. According to POSIX fdopendir [2]: Upon calling closedir() the file descriptor shall be closed. So we should avoid "close(dfd)" after "closedir(dir)". With threads, there could be a race where an innocent fd gets closed. But Python GIL seems to help hiding the issue well. [1]: http://pubs.opengroup.org/onlinepubs/009695399/functions/closedir.html [2]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fdopendir.html