ipv6 capable VPN provider pushing v6

[ThomasWaldmann]

cyberghost started pushing v4 and v6 today (previously: only v4).

this breaks openvpn 2.3 if v6 is disabled on all interfaces.
it exits with an error if it can not configure v6 on an interface.
there is no way in openvpn 2.3 to avoid this.

https://community.openvpn.net/openvpn/ticket/849

[ThomasWaldmann]

solution for vpngw:

add jessie-backports repo

apt install openvpn/jessie-backports # 2.4.0

add this to the openvpn configs:

pull-filter ignore "ifconfig-ipv6 "
pull-filter ignore "route-ipv6 "

[ThomasWaldmann]

unrelated changes:

the cyberghost provider side changes also required / enabled:

• certs and keys needed to be downloaded from CG again and reinstalled for all tunnels
• tls 1.2 works now \o/