Fix wp_guess_url() to work in every scenario I could find, allows us to use it to determine the correct path to the WordPress Site URL before installation for install.php and setup-config.php redirects. Fixes #24480 Fixes #16884

Built from https://develop.svn.wordpress.org/trunk@25396


git-svn-id: http://core.svn.wordpress.org/trunk@25327 1a063a9b-81f0-0310-95a4-ce76da25c4cd
* Fill in undefined var in `Tests_Option_BlogOption`
* Add `defined()` check for `BLOGSUPLOADDIR`
* Suppress deprecated function notices for `is_blog_user()` and `get_dashboard_blog()`
* Check existence of `$user` in `wpmu_log_new_registrations()` before arbitrarily making a database query

Fixes all notices in multisite unit tests.

See #25282.


Built from https://develop.svn.wordpress.org/trunk@25397


git-svn-id: http://core.svn.wordpress.org/trunk@25328 1a063a9b-81f0-0310-95a4-ce76da25c4cd