tailcfg: add MapResponse.ControlTime field

And log it when provided in map responses.

The test uses the date on which I joined Tailscale. :)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

control/controlclient/direct.go

@@ -772,6 +772,9 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 			go answerPing(c.logf, c.httpc, pr)
 		}
 
+		if !resp.ControlTime.IsZero() {
+			c.logf("netmap: control time is %v", resp.ControlTime.UTC().Format(time.RFC3339Nano))
+		}
 		if resp.KeepAlive {
 			vlogf("netmap: got keep-alive")
 		} else {