New format for UNIX data

New format for UNIX data

This moves the UNIX data into reserved fields of the WIM dentry.

This is theoretically less extensible than the previous format, which was
to add a specially-named alternate data stream entry to each file with
UNIX data.  However, having the UNIX data present in the metadata
resource is simpler and avoids problems when doing sequential extraction.

For now, this also seems to maintain compatibility with the MS
implementation, since it seems simply ignore the reserve fields.

Also, use 32-bit uids, gids, and modes.