Fix compatibility issues with directory streams

Fix compatibility issues with directory streams

Fix two related bugs where a WIM image metadata resource written by
wimlib could not be understood by some MS software versions:

- Never write an extra stream entry for the unnamed data stream of
  directories.  The concept of "unnamed data stream" isn't applicable to
  directories.  This fixes compatibility with DISM when the WIM image
  contains a directory with named data stream(s).  (I think DISM used to
  be okay with this, but on 10.0.20348.681 I'm seeing this issue.)

- Store the reparse point hash in main_hash when no extra stream entries
  are otherwise needed, i.e. when the file is a directory with no named
  data streams.  This fixes compatibility with the Windows 8 setup
  wizard, at least for some specific version of Windows 8, as reported
  at https://wimlib.net/forums/viewtopic.php?t=671.

Resolves https://wimlib.net/forums/viewtopic.php?t=671