ddk header cleanup

[Timo Kreuzer]

Now that Stefan has cleaned up some stuff in the ddk headers, maybe we
should think of some additional design fixes here.

We currently have ntddk.h include "per version" headers: winxp.h,
win2k.h, winnt4.h. These contain very few stuff.
My suggestion is to merge them into winddk.h. We could also merge
winddk.h and ntddk.h

It might make sense to split wdm.h and ntddk.h into architecture
specific files

Layout suggestion:
- ntddk.h: includes wdm.h and contains arch-independent definitions
currently in winddk.h/win2k.h/winnt4.h
- wdm.h: includes arch-dependent headers, contains arch-independend wdm
definitions
- ddk_x86.h / ddk_x64.h, ddk_arm.h, ddk_ppc.h / ddk_mips.h:
arch-dependent definitions for both wdm.h and ntddk.h definitions
(#ifdef _NTDDK_ ...)

Comments appreciated.

Timo