Move ms-load.php and ms-default-constants.php inclusion back to ms-settings.php to avoid breaking WP-CLI.

Use require_once() to allow for ms-settings.php to be included multiple times while testing.

props jeremyfelt.
see #27884.
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git-svn-id: http://core.svn.wordpress.org/trunk@28732 1a063a9b-81f0-0310-95a4-ce76da25c4cd