Fix --mtu-disc option with IPv6 transport

Fix --mtu-disc option with IPv6 transport

Socket configuration of MTU discovery was done unconditionally at IP level,
which has no effect for other protocols. This fixes the issue of OpenVPN
sending fragmented tcp6/udp6 packets even when 'mtu-disc yes' option is
passed.

Patch V2 (by Arne Schwabe): Rebase to current master and have
separate #ifdefs for IPv4 an IPv6

Signed-off-by: Julien Muchembled <jm@nexedi.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1444470291-2980-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10229
Signed-off-by: Gert Doering <gert@greenie.muc.de>