vhost: add 404 handler url option

This allows you to set a 404 handler URL on a vhost.

The necessary user code looks like...

    info.error_document_404 = "/404.html";

... at vhost-creation time.

In the existing lws_return_http_status() api, if it sees
the vhost has an "error_document_404" path set and that
we are trying to report a 404, it changes the action
instead to a redirect to the error_document_404 path.

The redirect target is returned using 404 status code.

If the redirect target doesn't exist, then it falls back
to just reporting the simple canned 404.