Added handle_t type to server interface so that we can make handles
pointers later on.
Always use 0 to signal invalid handle in server requests.

diff --git a/win32/device.c b/win32/device.c
index e3fbab9..307b2f5 100644
--- a/win32/device.c
+++ b/win32/device.c
@@ -331,7 +331,7 @@
 
     FIXME( "Unknown VxD %s. Try --winver nt40 !\n", filename);
     SetLastError( ERROR_FILE_NOT_FOUND );
-    return HFILE_ERROR;
+    return 0;
 }
 
 static const struct VxDInfo *DEVICE_GetInfo( HANDLE handle )