Warning when installing and uninstalling from base directory

This thread brought up a risk to user data when installing and uninstalling scripts into the base public_html directory:

http://www.virtualmin.com/forums/message-view?message_id=76521

We should warn users in some sort of way that installing into the base may overwrite any existing data in that directory, and that if the script is uninstalled it will also delete anything else in the directory.