Split fastcgi socket path and document root option and add the

Split fastcgi socket path and document root option and add the
SCRIPT_FILENAME CGI param with a prepended root.  This fixes php-fpm
that expects SCRIPT_FILENAME and also works with slowcgi if you
configure the root correctly.  For example, if SCRIPT_NAME and
REQUEST_URI are /php/index.php, root is /htdocs, SCRIPT_FILENAME will
be /htdocs/php/index.php.  As tested and discussed with florian@