Fire wp_auth_check_load() from admin_enqueue_scripts instead of admin_init so that it can access the current screen object.

Black list the update and upgrade screens.

Allow plugins to white/black list screens via the wp_auth_check_load filter.

Props nacin

see #23295


git-svn-id: http://core.svn.wordpress.org/trunk@24738 1a063a9b-81f0-0310-95a4-ce76da25c4cd