Pass source-link-node param to href.target when doing footnote processing

[sandersk]

When doing footnote processing, pass source-link-node param to all calls of href.target, e.g.:

https://github.com/oreillymedia/HTMLBook/blob/master/htmlbook-xsl/elements.xsl#L339
https://github.com/oreillymedia/HTMLBook/blob/master/htmlbook-xsl/elements.xsl#L377
https://github.com/oreillymedia/HTMLBook/blob/master/htmlbook-xsl/elements.xsl#L461

so that bidirectional hrefs for footnotes in the same chunk will be more concise, (e.g., href="#id123" instead of href="ch01.html#id123")