MFC r260571: fts: Stat things relative to the directory fd, if possible.

As a result, the kernel needs to process shorter pathnames if fts is not
changing directories (if fts follows symlinks (-L option to utilities), fts
cannot open "." or FTS_NOCHDIR was specified).

Side effect: If pathnames exceed PATH_MAX, [ENAMETOOLONG] is not hit at the
stat stage but later (opendir or application fts_accpath) or not at all.