Fix IPv6 routes on tap interfaces on OpenSolaris/OpenIndiana

Fix IPv6 routes on tap interfaces on OpenSolaris/OpenIndiana

The "route add" code always used "metric 0" on OpenSolaris, because
(on tun interfaces) it was required to make the route work on
"non-ethernet" interfaces (connected, no NDP).

This breaks routes via tap interfaces on recent Solaris versions
(tested on OpenIndiana 2019) - there, routes only work if metric
is != 0 (or just not set).  Otherwise it tries to map the gateway
address to a local address and fails.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20191009095200.9337-2-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18906.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 01b3a48c29df6dc4bade3d8fb7903346a3f77887)