Use unspecified, check unwanted properties

- Use `unspecified' instead of `nil' to represent empty options,
  hopefully this allows for better cooperation with themes loaded
  after this one.
- Specify any face properties that come from the face definitions that
  we don't want to use as `unspecified'.
- Change `diff-hl-*' to have a different foreground and background
  color, the effect created before this should be reproducible by
  setting `diff-hl-draw-borders' to nil.