- Do not probe paths that do not exist (ENOENT).
- If the OS supports open()ing directories (HAVE_FDOPENDIR), do not
  probe paths that cannot be opened.
- Do not probe files whereby fstat() fails.
- Do not probe non-regular files other than directories and block
  devices (previously, only FIFOs were discarded).
- Look for anchor even if fstat() is not supported.