Consolidated processing of all FO PIs into wrapper templates in

Consolidated processing of all FO PIs into wrapper templates in
one place (the fo/pi.xsl file) and added embedded documentation
for all of them.  This makes it possible to now include generated
documentation for the PIs in the output docs. Next stop: HTML PIs,
then on to the common ones...

This touched a lot of files and it is likely that I made some
mistakes in moving the PI processing over. Meaning that some PIs
might quit working or not work as expected after this change. So
it will need to tested. Which is why we do dot-zero releases...