Remove 'admin' as default username in install.

Props chrisrudzki
fixes #24078


git-svn-id: http://core.svn.wordpress.org/trunk@24998 1a063a9b-81f0-0310-95a4-ce76da25c4cd
phpdoc for $wp_admin_bar in admin-bar.php

Props jeremyfelt
see #24900


git-svn-id: http://core.svn.wordpress.org/trunk@24999 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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git-svn-id: http://core.svn.wordpress.org/trunk@25000 1a063a9b-81f0-0310-95a4-ce76da25c4cd