sshproxy-dumpd: protect traffic using encryption and client authentication

[dolmen]

The sshproxy-dumpd service v2.1.1 receives SSH session data in clear, and the client is not authenticated. That can be fine if the network is trusted (ex: localhost), but not in all settings.

Here is a gradual list of improvements proposals:

1. warn about this risk in shproxy-dumpd documentation (doc/sshproxy-dumpd.txt)
2. document a way to setup sshproxy and ssh-proxy-dumpd an encrypted tunnel with forward secrecy, as an external solution. Maybe using ssh tunneling?
3. implement encryption in the service itself (and the client) using TLS, with authentication of both server and client to each other