Fix FreeBSD ifconfig for topology subnet tunnels.

Fix FreeBSD ifconfig for topology subnet tunnels.

For "topology subnet", we only pretend to have a subnet and keep
using the tun if in point-to-point mode - but for that to fully
work, the "remote" address needs to be different from the "local"
address.  So just arbitrarily construct one from the on-link
subnet - base+1, if "that is not us", base+2, otherwise.

Fix trac #481

See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194745

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Tested-By: Anton Sayetsky <vsasjason@gmail.com>
Lazy-ACK-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1430071438-31675-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9606
(cherry picked from commit 60fd44e501f2002459a49c6c9bc64370ea26ca87)