Form fieldsets render their legend properly

A focused fidelity fix for a control on almost every form: the fieldset legend. A fieldset groups related fields inside a labelled border, and its legend is the label that sits on the border line itself, with the border broken behind the text. The engine used to draw a plain, unbroken border with the legend as an ordinary line of text below it, which read as a stray heading rather than a group label. Now the legend straddles the top border exactly the way the reference browser draws it: the border opens up behind the legend, the label sits on the line, and the grouped fields clear below it. This works for a normal legend and a larger one, and it is verified pixel for pixel against the reference browser.