Introduce `ms_load_current_site_and_network`. This is used by core during the multisite bootstrap process to populate the `$current_site` and `$current_blog` globals based on a requested domain and path.
Return values from this function inform `ms-settings.php` as to whether a page view should continue, `ms_not_installed()` should fire, or a redirect to a new location should occur.
This was previously a procedural block in `ms-settings.php`. Wrapping this code and providing specific return values allows us to write tests that do not rely on the manual and repeated inclusion of `ms-settings.php`.
This should not be used by plugins or themes. Please.
See #34941.
git-svn-id: https://develop.svn.wordpress.org/trunk@37475 602fd350-edb4-49c9-b593-d223f7449a82