New file msvcrt/excpt.h. Move some stuff out of winnt.h into it.

diff --git a/files/dos_fs.c b/files/dos_fs.c
index f3f3aae..ff756fc 100644
--- a/files/dos_fs.c
+++ b/files/dos_fs.c
@@ -35,6 +35,7 @@
 #include "ntddk.h"
 #include "options.h"
 #include "wine/server.h"
+#include "msvcrt/excpt.h"
 
 #include "debugtools.h"