Find out why $_GET['q'] is 'node' during updates, and fix.

[jenlampton]

While working on #287 I noticed that the "Front page" link had an active class. While tracking how the active class got added, I discovered that backdrop_is_front_page() was returning true on core/update.php.

This is because $_GET['q'] is 'node'. I'm not sure how or why this is the case, but it should be fixed.