netsh: Clear existing IPv6 DNS servers before configuring new ones

netsh: Clear existing IPv6 DNS servers before configuring new ones

When there are no IPv6 DNS published, the adapter state is not
sanitized and might contain IPv6 DNS server from a previous session.

netsh_ifconfig_options() clears DNS servers for IPv4 already.

Signed-off-by: Simon Rozman <simon@rozman.si>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20200924064452.1001-2-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21078.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit dd754221024cf60226ebaa679ec65ccc23f4e402)