Patch by Ax:
- the index link ("View this book's table of contents.") which pointed
to http://drupal/book instead of to ... this book's table of contents.
- it doesn't print "previous", "next", "up", "index" if there are no
"previous", "next", "up", "index"
- it doesn't print "index" if "up" /is/ the "index"
- it swaps the order of "up" and "index", ie. "up" is in the row above
"index", between "previous" and "next" now. this is more logical and the
way all tree-browsing apps (python doc, info, ...) do it.