Add systemd notify support

[cognifloyd]

It would be helpful to add systemd notify support to make it easier to daemonize the server and client processes.

The notification would happen once the tunnel is up and running, ready to forward the TCP traffic.
So, the server would notify when clients can start connecting, and clients would notify when the connection to the server has been established and forwarded ports can start accepting traffic.

We can do this with github.com/coreos/go-systemd/daemon.

import (
  "github.com/coreos/go-systemd/daemon"
)

func whatever() {
  // get ready to accept connections then tell systemd when ready
  daemon.SdNotify(false, "READY=1")
}