fts, getcwd, glob: audit for dirfd returning -1

fts, getcwd, glob: audit for dirfd returning -1

* lib/fts.c (opendir): Remove #define; no longer used.
(opendirat): New arg PDIR_FD.  All callers changed.
(fts_build, _opendir2): Use new opendirat to avoid the need for
dirfd, or for checking whether dirfd returns a negative value.
Don't use opendir; always use openat followed by fdopendir.
* lib/getcwd.c (__getcwd): Don't reset fd; fdopendir no longer clobbers
it.
* lib/glob.c (link_exists_p): Add comment explaining why dirfd never
returns -1 here.
* modules/fts (Depends-on): Remove dirfd.
* modules/getcwd (Depends-on): Likewise.