Multisite: Rework the upload space usage tracking code so as to be fully pluggable.

* Moves some admin only functions into wp-admin/includes/ms.php from wp-includes/ms-functions.php
 * Reworked the variable naming to be more in line with the Coding Standards
 * Introduced a new get_space_used() function instead of calculating it in multiple places.

Fixes #21181 props dllh and jkudish for inital work on this.


git-svn-id: http://core.svn.wordpress.org/trunk@21387 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/dashboard.php

@@ -1097,7 +1097,7 @@ function wp_dashboard_quota() {
 		return true;
 
 	$quota = get_space_allowed();
-	$used = get_dirsize( BLOGUPLOADDIR ) / 1024 / 1024;
+	$used = get_space_used();
 
 	if ( $used > $quota )
 		$percentused = '100';