Comment/pingback handlers don't check whether the post type supports comments

[solarissmoke]

wp-comments-post and XML-RPC don't check post type support for comments before accepting them - I think they should. We can't rely solely on $post->comment_status because that may not always be set to "closed" even if the post type doesn't support comments (see also #13473)

Maybe we could add a check to comments_open()?

Would require #17568.

I don't know if the lack of a supported UI for comments (which is what 'supports' means across the board) means that wp-comments-post.php should not be able to accept comments.