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.


git-svn-id: https://develop.svn.wordpress.org/trunk@25835 602fd350-edb4-49c9-b593-d223f7449a82