Updated XREF handling

This pull request overhauls the handling of text nodes and hrefs in `<a>` elements as follows:

* I've eliminated the xref-placeholder-overwrite-contents parameter.

* Moving forward, if an `<a>` element has `data-type="xref"` on it, then its text node will be updated. If an `<a>` element has another `data-type` (`data-type="link"`) or no `data-type`, then the text node will not be updated. There is one exception, however. If an `<a>` element doesn't have `data-type="xref"` on it, but does not have `data-type="link"`, and does have an empty text node, and does have an href that is not a Web URL, then the stylesheets will go ahead and try to add gentext.

* If an XREF has a Web URL as its `href` (e.g., `href="http://oreilly.com"`). This should never really happen (as `data-type="xref"` is intended for internal XREFs only), but if it does, the text node will be left untouched.

* As before, if the XREF `href` can't be resolved, the gentext will be 3 question marks, and an error will be thrown in the log output.

* The `href` attributes on `<a>` elements (aside from indexterms and footnoterefs, which are handled separately) will always be updated if they are internal cross-references. This ensures the href is accurate when HTMLBook files are consolidated for PDF or consolidated/rechunked for EPUB/Mobi.

Also fixed a bug related to XSLT 1.0 processor compatibility that I inadvertently introduced for XSLT 2.0 compatibility. Stylesheets are not currently forward-compatible with XSLT 2.0, but will be addressing that in a future PR.