Increase default height of classhelp windows from 400 to 600. If classhelp popup windows are really opened in a separate browser window instead of just a new tab, the window size is often too small, especially when the search frame for finding usernames is displayed. The original size was introduced in 2002 and might have been meant for 640x480 resolutions and the new size might still be too small for many use cases, but this conservative/small increase should not hurt people with small displays.