rtcp - A simple port forwarder (secured)

It setups an VPN over SSH, using similar techniques to gitolite.

Objective: Setup a tun/tap interface for the VPN

Server implementation notes:
1. rtcp_server listening to an UNIX socket, is the server part, 
   holds the tun/tap interface.
2. has an ssh user setup with the keys of the client in the authorized_keys,
   has the .hushlogin file in $HOME and /usr/bin/rtcp_helper as the shell
   for the user in "/etc/passwd".
3. the rtcp_helper connects to the server and forwards the traffic.

Client implementation notes:
1. rtcp_client is the client part.
2. Called as "rtcp_client rtcpuser@host".
3. Creates the client tun/tap interface and fordwars the traffic.