route.c: use sitnl to implement get_default_gateway_ipv6()

route.c: use sitnl to implement get_default_gateway_ipv6()

get_default_gateway_ipv6() has always been implemented using
netlink, however, now that we have sitnl, we can re-use the
latter and get rid of the netlink code from route.c.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190715144609.19616-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18667.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>