control/controlclient: replace a status func with Observer interface

For now the method has only one interface (the same as the func it's
replacing) but it will grow, eventually with the goal to remove the
controlclient.Status type for most purposes.

Updates #1909

Change-Id: I715c8bf95e3f5943055a94e76af98d988558a2f2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -101,6 +101,12 @@ type Direct struct {
 	lastPingURL  string // last PingRequest.URL received, for dup suppression
 }
 
+// Observer is implemented by users of the control client (such as LocalBackend)
+// to get notified of changes in the control client's status.
+type Observer interface {
+	SetControlClientStatus(Status)
+}
+
 type Options struct {
 	Persist              persist.Persist                    // initial persistent data
 	GetMachinePrivateKey func() (key.MachinePrivate, error) // returns the machine key to use
@@ -122,8 +128,9 @@ type Options struct {
 	Dialer               *tsdial.Dialer               // non-nil
 	C2NHandler           http.Handler                 // or nil
 
-	// Status is called when there's a change in status.
-	Status func(Status)
+	// Observer is called when there's a change in status to report
+	// from the control client.
+	Observer Observer
 
 	// SkipIPForwardingCheck declares that the host's IP
 	// forwarding works and should not be double-checked by the