IPv6 address/route delete fix for Win8

Use "store=active" for IPv6 address and route deletion - seems to be
required on Windows 8 and up, and not doing it will break OpenVPN
reconnection (old addresses are not properly deleted, thus address can
not be configured on connect).

Reported-by: Cedric <cedric+openvpn@bgtn.net>
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Cedric Tabary <cedric+openvpn@bgtn.net>
Message-Id: <20140413170648.GU16637@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8499