gupdate: Fix upgrade on FreeBSD

We did not use the correct command to call back the script to update the
ports with the default FreeBSD update method.
update-site: Add the script update-site

This script use xsltproc to convert pages from XML/XSLT to HTML. It uses
a cache to convert only the necessary pages. This eases the transfer to
the server using tools such as rsync or unison.