setting servername with an IPv6 request must ensure we have both brackets

setting servername with an IPv6 request must ensure we have both brackets

On lighttpd 1.4.28, the SERVER_NAME CGI variable is truncated at the first
colon. This makes it return an incorrect value for SERVER_NAME when the user
make the request to an IPv6, it outputs something like [2001.

This patch make sure we have either both opening and closing brackets or no
brackets at all (hence the 'xor' boolean check).

+ipv6