Support OpenSSH Layer 2 tunnels

[halhen]

Add support for OpenSSH Layer 2 TUN/TAP tunnels, established via the -w -oTunnel=ethernet options.

[halhen]

http://www.khanh.net/blog/archives/51-using-openSSH-as-a-layer-2-ethernet-bridge-VPN.html
http://fts.ifac.cnr.it/cgi-bin/dwww/usr/share/doc/openssh-server/README.tun.gz