Convert and rename the `$action` hook in wp-admin/admin-post to two dynamic hook sets.

* `admin_post_nopriv{$request_action}` is fired for logged-out users
* `admin_post{$request_action}` is fired for logged-in users

Props Otto42, DrewAPicture.
See #26869.

Built from https://develop.svn.wordpress.org/trunk@28349


git-svn-id: http://core.svn.wordpress.org/trunk@28177 1a063a9b-81f0-0310-95a4-ce76da25c4cd