do not push route-ipv6 entries that are also in the iroute-ipv6 list

do not push route-ipv6 entries that are also in the iroute-ipv6 list

A server should push a route to a client only if there is no matching
iroute for the same client.

While this logic works fine for IPv4, there is no IPv6 counterpart.

Implement the same check for IPv6 routes and discard matching ones
from the push list.

Trac: #354
Cc: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Message-Id: <20220628082024.19059-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24577.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>