Make the dashboard screen (index.php) occupy the same menu index across all...

Make the dashboard screen (index.php) occupy the same menu index across all admins (site/network/user).  Props wpmuguru. fixes #17751

git-svn-id: https://core.svn.wordpress.org/trunk@18839 1a063a9b-81f0-0310-95a4-ce76da25c4cd