user32: Do not allow a change of capture if the currently capture window is a menu unless explicitly specified

[Peter Dons Tychsen]

Hello,

Description:

Do not allow a change of capture if the currently capture window is a
menu unless explicitly specified. Handle failure in set_capture_window()
correctly in SetCapture() and ReleaseCapture(). 

- With tests.
- Tested on windows-xp
- Fixes bug http://bugs.winehq.org/show_bug.cgi?id=21212

Thanks,

/pedro