dvdnav: cleanup probing
- 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.