Establish remote proxy when SSH-tunneling to unix socket

We really don't want to break `fleetctl --tunnel` when we switch the default fleetctl driver to the fleet API. The fleet API is only deployed on /var/run/fleet.sock. This is a problem as the SSH forwarding protocol does not support tunneling over TCP to unix sockets. 

The approach of this PR is to establish an SSH tunnel to the target machine that hosts an scp-like proxy between the SSH session's stdin/stdout and a unix socket.