I haven't tried this so I can't say 100% that I understand how or if it works but it's worth a shot as what I remember from other posts GoDaddy won't allow you to install DNN in the root of your site.

Around Line 41 of your web.config is a line that states <add key="HostHeader" value=""/>. From the commented line below that it looks like you could leave your site setup at Domainname/dotnetnuke but if you put www.domainname.com as the value your URL address will show up as http://www.domainname.com. Then it's just a simple redirect from the root of your site. I don't know if you'll have to add a PortalAlias for www.domainname.com or not though.

Here's how I would go about testing this.
1. Setup the redirect in the root folder and make sure that it works.
2. Make the change in my web.config (If it works then I'm done otherwise go to Step 3)
3. If I couldn't get to my site anymore then I'd reverse the changes in my web.config.
4. Backup the database.
5. Add a PortalAlias for www.domainname.com.
6. Make sure everything is still working
7. Try the changes to the web.config again.
8. If everything works then great, if not then reverse the changes to the web.config and restore the database (if needed).