In automatic background updates, standardize on 'update'.

New, final filter names:
 * auto_update_{$type} (plugin, theme, core, language)
 * automatic_updates_is_vcs_checkout
 * automatic_updates_disabled

New class name is WP_Automatic_Updater. Method names include update() and should_update().

see #22704.

Built from https://develop.svn.wordpress.org/trunk@25835


git-svn-id: http://core.svn.wordpress.org/trunk@25747 1a063a9b-81f0-0310-95a4-ce76da25c4cd
set_current_screen() is now unnecessary in Automatic_Upgrader_Skin, which is good because it breaks things in the network admin.

fixes #25622.

Built from https://develop.svn.wordpress.org/trunk@25836


git-svn-id: http://core.svn.wordpress.org/trunk@25748 1a063a9b-81f0-0310-95a4-ce76da25c4cd