(change_file_owner): Use ent->fts_statp only if

(change_file_owner): Use ent->fts_statp only if
needed.  Chown a directory only after chowning its children; this
avoids problems if the new directory ownership doesn't permit
access to the children.  Dereference symlinks before doing
ROOT_DEV_INO_CHECK, not after, so that we catch symlinks to /.
Do not optimize away the chown() system call when the file's owner
and group already have the desired value.  POSIX does not permit
this optimization.  Rely on chown and lchown to do the right
thing with symlinks and/or -1 arguments, now that we have wrappers
to do this.  Use ENOTSUPP not ENOSYS, and ignore all ENOTSUPP
errors, not just command-line errors.
(chown_files): Pass FTS_STAT to xfts_open if we don't need file status.