the first entry of the zip central directory.
  Here we require the mmapped block to represent a real zip file where the
  disk_trailer is _last_ in the file area, so that its position would be at
  a fixed offset from the end of the file area if not for the comment field
  allowed to be of variable length (which needs us to do a little search
  for the disk_tailer). However, in this simple implementation we disregard
  any disk_trailer info telling about multidisk archives, so we just return
  a pointer to