Make a virtual link to the latest source called latest.tar.gz

Make a virtual link to the latest source called latest.tar.gz

The idea came from Rich Salz and Stefan Eissing <stefan.eissing@greenbytes.de>
and is that it would be nice if a script could ask for /source/latest.tar.gz
and get a 302 that points at the actual latest tarball.

For this to become as smooth as possible, it required three things:
- add some markup around the latest tarball (a <span class="latest"> wrapper),
  done in openssl.wml.
- Move source/.htaccess to source/.htaccess.in and add an appropriate rewrite
  to implement the 302, with a visible marker indicating where the file name
  of the currently latest tarball should be inserted.
- do a bit of hackery in Makefile to extract the latest file name from
  source/index.html after that one has been rebuilt, and use that to create
  source/.htaccess from source/.htaccess.in.