- Add idempotency guards so the structures can be used in other
  utilities.
- Update bpb structs with reserved fields.
- In direntry struct join deName with deExtension. Although a
  fix was attempted in the past, these fields were being overflowed,
  Now this is consistent with the spec, and we can now share the
  WinChksum code with NetBSD.

Submitted by:	Pedro F. Giffuni <giffunip tutopia com>
Mostly obtained from:	NetBSD
Reviewed by:	bde
MFC after:	2 weeks