 the zip central directory.
</p><p>
  For an actual means, we are going to search backwards from the end
  of the mmaped block looking for the PK-magic signature of a
  disk_trailer. If we see one then we check the rootseek value to
  find the first disk_entry of the root central directory. If we find
  the correct PK-magic signature of a disk_entry over there then we
  assume we are done and we are going to return a pointer to that label.
</p><p>
  The return value is a pointer to the first zzip_disk_entry