repair tap mode on OpenSolaris/OpenIndiana

repair tap mode on OpenSolaris/OpenIndiana

commit 611fcbc48 joined the two distinct calls for "add ipv6 address
to tap interface" and "set MTU" for Solaris - but due to peculiarities
on how this platform handles IPv6 addresses ("ifconfig addif" creates
a new "tap0:1" subinterface with the new address - which does not have
a distinct MTU) this does not work.

un-join calls again.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20191009095200.9337-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18905.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>