cmd/tailscale/cli: add ping subcommand

For example:

$ tailscale ping -h
USAGE
  ping <hostname-or-IP>

FLAGS
  -c 10                   max number of pings to send
  -stop-once-direct true  stop once a direct path is established
  -verbose false          verbose output

$ tailscale ping mon.ts.tailscale.com
pong from monitoring (100.88.178.64) via DERP(sfo) in 65ms
pong from monitoring (100.88.178.64) via DERP(sfo) in 252ms
pong from monitoring (100.88.178.64) via [2604:a880:2:d1::36:d001]:41641 in 33ms

Fixes #661

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/message.go

@@ -33,6 +33,10 @@ type FakeExpireAfterArgs struct {
 	Duration time.Duration
 }
 
+type PingArgs struct {
+	IP string
+}
+
 // Command is a command message that is JSON encoded and sent by a
 // frontend to a backend.
 type Command struct {
@@ -56,6 +60,7 @@ type Command struct {
 	RequestEngineStatus   *NoArgs
 	RequestStatus         *NoArgs
 	FakeExpireAfter       *FakeExpireAfterArgs
+	Ping                  *PingArgs
 }
 
 type BackendServer struct {
@@ -148,6 +153,9 @@ func (bs *BackendServer) GotCommand(cmd *Command) error {
 	} else if c := cmd.FakeExpireAfter; c != nil {
 		bs.b.FakeExpireAfter(c.Duration)
 		return nil
+	} else if c := cmd.Ping; c != nil {
+		bs.b.Ping(c.IP)
+		return nil
 	} else {
 		return fmt.Errorf("BackendServer.Do: no command specified")
 	}
@@ -254,6 +262,10 @@ func (bc *BackendClient) FakeExpireAfter(x time.Duration) {
 	bc.send(Command{FakeExpireAfter: &FakeExpireAfterArgs{Duration: x}})
 }
 
+func (bc *BackendClient) Ping(ip string) {
+	bc.send(Command{Ping: &PingArgs{IP: ip}})
+}
+
 // MaxMessageSize is the maximum message size, in bytes.
 const MaxMessageSize = 10 << 20