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.

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git-svn-id: http://core.svn.wordpress.org/trunk@25747 1a063a9b-81f0-0310-95a4-ce76da25c4cd