Great changes in all the stylesheets. Every page is now generated in two

Great changes in all the stylesheets. Every page is now generated in two
phases:

1) A stylesheet from XML to XHTML that generates the page.
   This stylesheet no more bothers with the layout of the page w.r.t. the
   rest of the site (i.e. it doesn't add the link to CSS; it doesn't call
   the head and foot templates)
2) A common stylesheet from XHTML to HTML that just adds the header, footer
   and CSS link to the page.

The idea is that in this way it is easier to change the overall layout of
the site (e.g. removing frames). Moreover, less code is required.

Problem: xsltproc is too intelligent (= rather stupid), so that it
recognises XHTML pages and fails in applying the stylesheet. So, in the
Makefile, care (= sed) must be taken to remove the DOCTYPE definitions
from XHTML files.