In `comment_form()`, allow `name_submit` to be passed to change the `name` attribute of the submit button.

Props obenland.
Fixes #22792.

Built from https://develop.svn.wordpress.org/trunk@28813


git-svn-id: http://core.svn.wordpress.org/trunk@28620 1a063a9b-81f0-0310-95a4-ce76da25c4cd