attempt to add IPv6 route even when no IPv6 address was configured

attempt to add IPv6 route even when no IPv6 address was configured

Even if no IPv6 address is configured, OpenVPN still supports
transporting IPv6 segments, therefore adding an IPv6 route
should always be allowed.

However, the route might fail to be installed or may just not work
as expected, therefore, a proper warning should be printed to inform
the user of the possible pitfall.

Always allow adding an IPv6 route and print a WARNING when
no IPv6 address is configured for the interface.

Trac: #832
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170131112131.13570-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13994.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>