make "ipv6 ifconfig" on linux compatible with busybox ifconfig

make "ipv6 ifconfig" on linux compatible with busybox ifconfig

We used to call "ifconfig tun0 inet6 add...".  The "inet6" part is
optional,
and not understood by busybox.  So now we call "ifconfig tun0 add ...",
which works on all supported Linux variants.

Tested on Gentoo, RHEL5+, Debian Lenny & up.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: 1347364615-26878-1-git-send-email-gert@greenie.muc.de
URL: http://article.gmane.org/gmane.network.openvpn.devel/7052
Signed-off-by: David Sommerseth <davids@redhat.com>