Only show Comments are Closed when they are supported.

Tidy up the logic to be clearer.
Mark attachements as supporting comments.
Fixes #17568 props duck_

git-svn-id: https://develop.svn.wordpress.org/trunk@18211 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/post.php

@@ -55,6 +55,7 @@ function create_initial_post_types() {
 		'rewrite' => false,
 		'query_var' => false,
 		'show_in_nav_menus' => false,
+		'supports' => array( 'comments' ),
 	) );
 
 	register_post_type( 'revision', array(