- node system:
    + fixed a typo in node_load(): it should be faster now

- book module:
    + removed the functions book_parent() and book_parent_query() as
      they were no longer needed.  Gerhard & co: this should fix the
      occasional SQL errors you get, and should improve performance.

    + made the "next", "previous" and "up" links work correctly ...

    + XHTML-ified the code

    + added some missing translations

  I'm working on the book module now to make it possible to update book
  pages.