delay initialization until plugins loaded

If you have the plugin network activated and you're using SSL for the admin,
loading any admin page will give a notice that the global `$pagenow` is not
defined.

Notice: Undefined index: pagenow in /srv/www/default/application/wp/wp-includes/link-template.php on line 2451

The global is setup after network-active plugins load, but before blog
plugins load, and it's used in `get_home_url()` on SSL requests.