Explicitly globalize $blog_id, $public, and $site_id in ms-settings.php

[danielbachhuber]

See ​https://github.com/wp-cli/wp-cli/pull/2131

Related #23685

[jeremyfelt]

$blog_id as an almost explicit global was introduced in [3011] and moved to its current less explicit location in [12732]. I think we'll need to mark it right before calling wp_initial_constants() in wp-settings.php.

I don't remember ever reading this inline comment:

 * The $blog_id global, which you can change in the config allows you to create a simple
 * multiple blog installation using just one WordPress and changing $blog_id around.

34217.diff addresses the $blog_id issue.

$public was merged over with MU. I'm not sure if it was ever used by core, but it is no longer. I think we can get away with not explicitly globalizing it and we should instead think about the consequences of removing it. :)

$site_id was also merged over with MU. It is no longer used anywhere by core either. We should think about the consequences of removing this as well.

Related: we accidentally (maybe?) got rid of the $sites global in 3.9 and there were no explosions, just a desire to have a replacement via wp_get_networks() in #29415.

[jeremyfelt]

In 34961:

Make $blog_id explicitly global.

$blog_id is used in both single and multisite configurations as a global variable, though has never been explicitly marked as such in the global scope. This can cause confusion depending on how core is loaded.

Fixes #34217.