Prevent DNS leaks on Windows 10

- This will generate a warning in unsupported environments.
- This will not work if the client is using an OpenVPN version lower
than 2.3.9
- For OpenVPN 2.3.3+, ignore-unknown-option could be used instead of
setenv opt to prevent a warning.

TL;DR: upgrade to the latest OpenVPN on Windows, ignore the warning
elsewhere.

Thanks a lot for your continuous work on OpenVPN, @ValdikSS.

openvpn-install.sh

@@ -361,6 +361,7 @@ persist-key
 persist-tun
 remote-cert-tls server
 comp-lzo
+setenv opt block-outside-dns
 verb 3" > /etc/openvpn/client-common.txt
 	# Generates the custom client.ovpn
 	newclient "$CLIENT"