Fixes a regression introduced by 8d7de32cd646f3d1f118836e643e6146c9837278.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=59034
-- v5: win32u: Clear STARTF_USESHOWWINDOW in MessageBoxIndirectW(). user32/tests: Add tests showing that MessageBox() resets STARTF_USESHOWWINDOW. win32u: Implement NtUserModifyUserStartupInfoFlags(). win32u: Fetch startup info flags during initialization. win32u: Ignore statup cmd show mode for owned windows.