Start implementing custom ssh server

Turns out SSH has robust semantics for opening generic channels.
Looks like I'll be able to set up tunnels without ever needing to
forward ports on the server, since I can connect the channels with
a custom protocol.

Of course I'll eventually want to support generic SSH clients, but
this makes starting much easier.

boringproxy.go

@@ -34,6 +34,7 @@ type BoringProxy struct {
 	tunMan        *TunnelManager
 	adminListener *AdminListener
 	certConfig    *certmagic.Config
+        sshServer     *SshServer
 }
 
 func NewBoringProxy() *BoringProxy {
@@ -67,7 +68,9 @@ func NewBoringProxy() *BoringProxy {
 
 	auth := NewAuth()
 
-	p := &BoringProxy{config, auth, tunMan, adminListener, certConfig}
+        sshServer := NewSshServer()
+
+	p := &BoringProxy{config, auth, tunMan, adminListener, certConfig, sshServer}
 
 	http.HandleFunc("/", p.handleAdminRequest)
 	go http.Serve(adminListener, nil)