Update `docbook' rendition backend:
    
        - Split PDF output. There is now a PDF outupt produced from
          DocBook XML (`docbook_convertToPdfFromSgml') and one PDF output
          produced from DocBook SGML (`docbook_convertToPdfFromXml').
    
        - Add `docbook2pdf.dsl' to customize output produced from
          SGML-based DocBook files. This file was taken from
          /usr/share/sgml/docbook/utils-0.6.14/docbook-utils.dsl.
    
        - Rname function name from `docbook_prepareXsl4Using' to
          `docbook_prepareStyles'.
    
        - Update variable names to reflect the fact we aren't using xsl
          files only, but styles.