ms-admin.php stats should use the available cached data + count functions should exclude partially registered users/blogs from stats

[Denis-de-Bernardy]

There are two calls at the top of the file:

$c_users = $wpdb->get_var("SELECT COUNT(id) FROM {$wpdb->users}");
$c_blogs = $wpdb->get_var("SELECT COUNT(blog_id) FROM {$wpdb->blogs}");

but in ms-functions.php, there are several stat functions that could be used and/or improved so that we access (at least partially) cached data.

[Denis-de-Bernardy]

In particular get_user_count() and get_blog_count(). Given the nature of the two strings immediately isn't safe to assume that we want the number of *active* users and blogs rather than the raw number of users -- including spammers and partially registered users.

[wpmuguru]

(In [13114]) use cached data in ms-admin.php stats, props ddebernardy, fixes #11772