fts, savedir: avoid glibc 2.2 readdir ENOENT bug

fts, savedir: avoid glibc 2.2 readdir ENOENT bug

This is mostly to document the bug.
If these old platforms were still common I suppose we should
change the readdir module to work around it.  However, I’m not
sure it’s worth the hassle at this point.
* doc/posix-functions/readdir.texi, doc/posix-functions/readdir_r.texi:
Document the bug.
* lib/fts.c (fts_build):
* lib/savedir.c (streamsavedir):
Work around it.