allow usage of --server-ipv6 even when no --server is specified

allow usage of --server-ipv6 even when no --server is specified

Until now OpenVPN has not allowed to specify --server-ipv6
if no --server was also set. This constraint comes from the
fact that most of the IPv6 logic (i.e. ifconfig-pool handling)
relied on IPv4 components to be activated and configured as
well.

Now that the IPv6 code path has been made independent from
IPv4, it is finally possible to to relax the constraint
mentioned above and make it possible for the user to have a
configurations with --server-ipv6 only.

Trac: #208
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200530000600.1680-4-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19949.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>