Option to process xincludes in body, script

The attached patch updates xmlcontent to enable optional Xinclude processing on the input, the script, or both. The patch updates both the UI and the processing chain.

On the script side, this helps to support modular XSLT scripts, e.g. DocBook/XML, which might turn out to be popular if available.

On the body side, this provides a path to enable (among other things) pass-through of pre-existing static XML or HTML pages that can be pulled from some existing place -- either from the local file system or from a URL. By building an appropriate XSLT script, the pulled HTML can start out arbitrarily rich and be down-filtered to whatever is appropriate for Drupal content display. This is not an appropriate mechanism for import, because it cannot resolve links that exit the document. A suitably careful transform, however, should be able to remove external links if desired. General import is challenging enough that a simpler approach like this is sometimes useful.

Note that include processing on the body is NOT enabled by default. See the tip string on that for an explanation of why that is so.