win32u: Use user_callbacks for alertable NtWaitForMultipleObjects calls.

dlls/user32/user_main.c

@@ -162,6 +162,7 @@ static const struct user_callbacks user_funcs =
     EndMenu,
     ImmProcessKey,
     ImmTranslateMessage,
+    NtWaitForMultipleObjects,
     free_win_ptr,
     MENU_GetSysMenu,
     MENU_IsMenuActive,

dlls/win32u/driver.c

@@ -813,8 +813,11 @@ static NTSTATUS nulldrv_MsgWaitForMultipleObjectsEx( DWORD count, const HANDLE *
 {
     if (!count && timeout && !timeout->QuadPart) return WAIT_TIMEOUT;
 
+    if (!user_callbacks)
         return NtWaitForMultipleObjects( count, handles, !(flags & MWMO_WAITALL),
                                          !!(flags & MWMO_ALERTABLE), timeout );
+    return user_callbacks->pNtWaitForMultipleObjects( count, handles, !(flags & MWMO_WAITALL),
+                                                      !!(flags & MWMO_ALERTABLE), timeout );
 }
 
 static void nulldrv_ReleaseDC( HWND hwnd, HDC hdc )

dlls/win32u/ntuser_private.h

@@ -35,6 +35,7 @@ struct user_callbacks
     BOOL (WINAPI *pEndMenu)(void);
     BOOL (WINAPI *pImmProcessKey)(HWND, HKL, UINT, LPARAM, DWORD);
     BOOL (WINAPI *pImmTranslateMessage)(HWND, UINT, WPARAM, LPARAM);
+    NTSTATUS (WINAPI *pNtWaitForMultipleObjects)(ULONG,const HANDLE*,BOOLEAN,BOOLEAN,const LARGE_INTEGER*);
     void (CDECL *free_win_ptr)( struct tagWND *win );
     HMENU (CDECL *get_sys_menu)( HWND hwnd, HMENU popup );
     HWND (CDECL *is_menu_active)(void);