Make comments_open and pings_open filterable

[tellyworth]

This makes it possible to filter the result of the comments_open and pings_open post status flags. It also changes wp-comments-post.php and wp-trackback.php to call those functions rather than checking $post->comment_status and $post->ping_status directly. This makes it possible for plugins to override and disable comment posting.

Unfortunately many (most?) themes check those flags directly rather than calling comments_open() and pings_open(), so they might incorrectly display a comment form when comments are in fact disabled.

(In [6716]) Add filters to comments_open() and pings_open(). Props tellyworth. fixes #5761