this patch allows to generate valid html4 and xhtml.
It appears the only difference is the '<input>' element,
which must contain a closing '/' for xhtml, but not for
html4.

The patch thus encapsulates the generation of this
element in two functions (one per html type), and
the relevant classes then refer to the desired version
depending on the 'HTML_VERSION' variable in the
config file. (if it's undefined, 'html4' is assumed).

Most other changes were in the html templates. I'd
suggest to fork the "classic's" html templates, and
let one be html4, the other xhtml compliant.

I'm willing to provide the patches to get this new
cgi interface in sync with xhtml.