HTML Editor in file manager will not work in proxied situations

Whilst investigating the cause of Bug #688 (http://www.virtualmin.com/bug-tracker/bug?bug%5fnumber=688) I found that the new HTML editor included in the File Manager module will not work in situations where Webmin is accessed within a subdirectory of another web server (i.e. Apache using mod_proxy).

Line 14 of edit_html.cgi reads:
_editor_url = "/$module_name/htmlarea/";

For a proxy/directory safe implementation line 14 should read:
_editor_url = "$gconfig{'webprefix'}/$module_name/htmlarea/";

This will correctly load the HTML editor code no matter which subdirectory Webmin maybe located in.