Changes to automatic background updates in preparation for Beta 1.

 * Show a notice for beta testers on update-core.php explaining the status of their install. Three possibilities: auto updates are enabled, auto updates are disabled because the install doesn't support SSL HTTP requests, and auto updates are disabled because it is a VCS checkout.
 * Improve the output of the email, for maximum debugging potential. Failures are clearly labeled and the email leads testers to the support forums and Trac.
 * Try to create wp-content/languages in the upgrader if it doesn't exist. Our mkdir isn't recursive, so trying to create wp-content/languages/plugins could fail.
 * Abstract out version control checkout determination into a public method. The filter is now auto_upgrade_is_vcs_checkout, still subject to change.

see #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25633 602fd350-edb4-49c9-b593-d223f7449a82