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
Revert [28349] in favor of retaining the single dynamic `admin_post*` hook.

See #26869.

Built from https://develop.svn.wordpress.org/trunk@28350


git-svn-id: http://core.svn.wordpress.org/trunk@28178 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Rename the `$action` hook in wp-admin/admin-post.php to `admin_post{$action}`.

Also, clarify documentation of priv vs nopriv prefixing of the hook name.

See #26869.

Built from https://develop.svn.wordpress.org/trunk@28351


git-svn-id: http://core.svn.wordpress.org/trunk@28179 1a063a9b-81f0-0310-95a4-ce76da25c4cd