Platform cleanup for FreeBSD

Platform cleanup for FreeBSD

- cleanup TUN/TAP devices at program end ("ifconfig ... destroy")
- make TUN device setup for "topology subnet" work together with IPv6
  (setup correct netmask and route, but do not use IFF_BROADCAST)

There's one catch for FreeBSD 8.2 if you use pf(4): it will block IPv6
fragments by default, so the standard t_client.sh test sets fail unless
you specifically add "pass in on tun1 fragment" rules - but there's
nothing OpenVPN can do about it.

Tested with IPv4 and IPv6 on 7.4-RELEASE/amd64 and 8.2-RELEASE/amd64

Signed-off-by: Gert Doering <gert@greenie.muc.de>
URL: http://thread.gmane.org/gmane.network.openvpn.devel/5303
Acked-by: David Sommerseth <davids@redhat.com>
Signed-off-by: David Sommerseth <davids@redhat.com>