Windows: Restructure DDK/MSVC project files

For simplicity and to address some build warnings, this commit reworks
the project files to eliminate duplication and consolidate common build
flags and options. Also adds the stress and testlibusb projects to the
DDK build script.

Closes #402

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
diff --git a/msvc/fxload_sources b/msvc/fxload_sources
index d6e31d6..9f5dbd9 100644
--- a/msvc/fxload_sources
+++ b/msvc/fxload_sources
@@ -2,7 +2,7 @@
 TARGETTYPE=PROGRAM
 386_STDCALL=0
 
-_NT_TARGET_VERSION= $(_NT_TARGET_VERSION_WINXP)
+_NT_TARGET_VERSION=$(_NT_TARGET_VERSION_WINXP)
 
 !IFNDEF MSC_WARNING_LEVEL
 MSC_WARNING_LEVEL=/W3