Route uploads through async-upload.php instead of admin-ajax.php

[koopersmith]

wp.Uploader sends its uploaders to admin-ajax through the wp_ajax_upload_attachment() method instead of to async-upload. While the resulting code is cleaner, some installs may have async-upload whitelisted and/or provisioned differently than admin-ajax, resulting in errors (including but not limited to differing file size limitations).

After discussing ​in IRC, we're going to route everything through async-upload for 3.5.

Not working yet, but adds the code to make plupload send through async-upload.

Working.

22622.4.diff works well here too.

In 22902:

Route uploads through async-upload.php instead of admin-ajax.php.

Props nacin, koopersmith
fixes #22622