In ES5 mode, emit rewritten CSS inside the guest document.

In ES5 mode, or document.write content in ES5/3 mode:
* <style> elements in the guest document contain rewritten CSS rather
  than being empty.
* <link rel=stylesheet> elements' rewritten CSS goes into <style>
  elements inserted immediately before them. This is unfortunately
  visible to the guest, but I expect to be unlikely to be noticed.
* <link> elements get normal element processing rather than being
  discarded after having stylesheet-loading side effects.

This is preparation for being able to use an iframed document as a vdoc
container, where there is no enclosing document to create style elements
in.

@r5459

[felix8a, 2013-06-24 18:18:42]

lgtm