make_icon_filename(): Convenience function to turn a partial filename

into a usable filename using $ICONSERVER and
                       $IMAGE_TYPE as needed.

get_link_icon():  Function to examine a URL and return the string to use
                  to insert an icon if the link points off-site, if needed
                  and $OFF_SITE_LINK_ICON is set.

Adjusted appropriate places to use these new functions.