Restore the parts of fts that were removed on 2006-01-17, so that

Restore the parts of fts that were removed on 2006-01-17, so that
it's easier for legacy applications designed for the version
of fts in glibc or BSD to convert to this more robust version.
Add a new mode, FTS_CWDFD, by which to enable the improved
(openat- based -- aka no-chdir) semantics.

* fts_.h (FTS_CWDFD): Define.  Callers must use this fts_open
option to enable the more robust behavior.
(FTS_OPTIONMASK): Widen accordingly.
* fts.c: Restore removed code, reverting the default behavior.