Core Auto Updates: Add a initial skin to allow capturing the output from the upgrader without displaying it (such as we need during cron calls).
This has been copied almost verbatim from the automatic-updater plugin with a few style tweaks and additional upgrade-possible check. See #22704

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git-svn-id: http://core.svn.wordpress.org/trunk@25198 1a063a9b-81f0-0310-95a4-ce76da25c4cd