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.