Option to explicitly redirect the homepage

[svandragt]

With WordPress if you want the homepage to be not found, you have to

1. change the "Your homepage displays" setting in Settings > Reading to "a static post". Otherwise WP will search the template hierarchy and attempt to display any existing post.
2. unpublish the static post, to return a 404.

The interface displays this state incorrectly, where a page is set, but it appears as if no page is set:

It would be nicer to have a third radio button to override this behaviour and return the homepage as a 404, regardless of the setting.
Also show this on the plguni settings page.