Improve IPv6 handling

- When the server does not have a public IPv6 address, add "block-ipv6"
  to the client configuration to help prevent IPv6 leaks. This option
  is supported in OpenVPN client versions 2.5.x and newer.
- Closes #13. Thanks @do02fw for the suggestion.

openvpn-install.sh

@@ -656,6 +656,7 @@ topology subnet
 server 10.8.0.0 255.255.255.0" > /etc/openvpn/server/server.conf
 	# IPv6
 	if [[ -z "$ip6" ]]; then
+		echo 'push "block-ipv6"' >> /etc/openvpn/server/server.conf
 		echo 'push "redirect-gateway def1 bypass-dhcp"' >> /etc/openvpn/server/server.conf
 	else
 		echo 'server-ipv6 fddd:1194:1194:1194::/64' >> /etc/openvpn/server/server.conf
@@ -808,7 +809,7 @@ persist-tun
 remote-cert-tls server
 auth SHA256
 cipher AES-128-GCM
-ignore-unknown-option block-outside-dns
+ignore-unknown-option block-outside-dns block-ipv6
 verb 3" > /etc/openvpn/server/client-common.txt
 	# Enable and start the OpenVPN service
 	(