Improve wp_list_comments() markup.

* Whitespace cleanup
* Accepts format=html5 which uses some of the HTML5 elements
* Some helpful HTML comments to help untangle the HTML
* Other misc code cleanup

props lancewillett, obenland, georgestephanis. fixes #20088.

git-svn-id: https://core.svn.wordpress.org/trunk@23694 1a063a9b-81f0-0310-95a4-ce76da25c4cd