Fixes #10674: Form controls in disabled fieldset legend

This undoes the disabling of all form controls for those within the **first** legend in a disabled fieldset. It's a gnarly selector, and doesn't work for IE8, so I'll need to add a note to the docs, but it does the trick. See it in action: http://jsfiddle.net/tyVF4/3/.

/cc @darobin @cvrebert