On a Windows box joined to a domain, GetUserName will not return the domain
part. On a Unix box joined to a domain via winbindd, wine_get_user_name will.
So we need to cut off the domain instead of just replacing the \ character.

This are internal helper functions; the API functions that use them
already use LONG.

user32: Use a simplified sequence to destroy windows at thread exit, since we don't want to send messages.