Enable an fts optimization (call lstat only for directories,

Enable an fts optimization (call lstat only for directories,
on some file system types) also with the --preserve-root option
of chown or chgrp.
* src/chown-core.c (change_file_owner): Compare fts_statp-based
dev/ino against root dev/ino only for directories.
(chown_files): Don't let the root_dev_ino setting influence whether
we use FTS_NOSTAT: fts always sets *fts_statp for a directory.