Push an IPv6 CIDR mask used by the server, not the pool's size

Push an IPv6 CIDR mask used by the server, not the pool's size

Correctly handle CIDR masks when pushing clients addressing from an IPv6
pool. This change ignores the incorrectly used `bits` argument to the
--ifconfig-ipv6-pool option.

The code to save any provided CIDR mask after the pool IP is left in;
this may someday become useful when we move to allow IPv6 pools without
relying on an IPv4 pool assignment.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <53F1DA95.7020701@usa.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8990
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c4ed931a70502a351ff1089aa1bfb8001586f788)