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


git-svn-id: https://develop.svn.wordpress.org/trunk@25228 602fd350-edb4-49c9-b593-d223f7449a82