Fix option#selected and select#selectedIndex, fixes #568 & #569

Thoroughly test select elements, and nail down the obvious expected
behavior that's not tested by the w3c tests.

  The addition to level2/html.js#702 makes sure selected options return
the index of the actual selected option, not the defaultSelected option

  The addition of the check for the `multiple` attribute at
level2/html.js#830 ensures that multi-select elements don't select
the first element by default.