Fix for EXT-6663 (TOS Screen logic for enabling agreement checkbox is questionable)

* remove the use of the start_url parameter, and instead store the loading url as a string in the xui, so that there's no chance of the loading screen page completing before we add our observer.
* replace the "load complete count" with explicit state variables for whether the loading screen has completed and whether the site is alive
* don't begin trying to load the real URL until both of the above conditions are met (and begin the load no matter which order they are satisfied in)

Reviewed by Richard at http://codereview.lindenlab.com/1218007