provide a comments closed equivalent for POSTs to wp-comments-post.php

The Facebook plugin for WordPress provides the opportunity to manage a post's comments through the [Comments Box social plugin](https://developers.facebook.com/docs/reference/plugins/comments/) stored and sorted on Facebook servers. The Comments Box removes the WordPress comment form from the page but does not currently reject new comments POSTed to [wp-comments-post.php](http://core.trac.wordpress.org/browser/trunk/wp-comments-post.php) for a post under its comments system.

Check if a new WordPress comment is submitted against a post with comments managed by the Facebook Comments Box social plugin. Reject these new comment attempts in the same way WordPress would reject the comment if the post was closed to new comments.