WP-CLI compatibility issues

There are 3 main issues (differences between web request and WP-CLI):

* Global variables need to be explicitly globalized [in order to function as expected in WP-CLI](https://github.com/wp-cli/wp-cli/issues/4019).
* WP-CLI doesn't load `advanced-cache.php`.
* `$_SERVER` variables ( HTTP_HOST, REQUEST_URI, HTTP_USER_AGENT, , ... ) are empty.

Until now, we have merged Automattic/wp-super-cache#384 (prevent creating output buffer for php-cli commands), Automattic/wp-super-cache#499 (fixes wp-cli plugin deactivate/activate ), Automattic/wp-super-cache#576 (fixes infinite amount of semaphore), Automattic/wp-super-cache#191 (avoids a PHP notice), Automattic/wp-super-cache#465 (move functions in phase1 file into phase2 file and load that everywhere).

Related issues: Automattic/wp-super-cache#586, Automattic/wp-super-cache#569, Automattic/wp-super-cache#498, Automattic/wp-super-cache#440, Automattic/wp-super-cache#431, Automattic/wp-super-cache#383, Automattic/wp-super-cache#243, Automattic/wp-super-cache#7.

The issue names WP-CLI behavior, advanced-cache.php, and $_SERVER variables as starting points; reproduce each difference by comparing a WP-CLI command with a web request, then review the linked WP-CLI and wp-super-cache issues. Done means the three compatibility gaps have agreed fixes and their behavior is verified in both execution contexts.

Written by the indexing model from the issue text.