Logtunnel

[sabban]

Add tunnel feature.

• This would close features discussions moul/sshportal#23
• This add tunnel feature.
• A slightly light issue with the select "loop" in bastionsession. This is solved by calling the whole ChannelHandler in a goroutiner in order multiple Channel be handled at the same time.
• Initially I gave a try to log tunnel with pcap format. But this can only be done by creating a custom network protocol or create a false tcp session (creating ACK, SYN+ACK packets from nothing, and put payload with PSH). So I tweaked a little the ttyrec format with name of host after the date, to see it with ttyplay. At least this allow ttyplay to replay session, and we are able to see session when they are human readable.
• I created a new pkg logtunnel after logchannel. I choose to create a new pkg, because I believe that our file format for logtunnel can evolve to a better thing, and this way this can be handled separately from the channel.