ipn, wgengine/magicsock: be more idle when in Stopped state with no peers

(Previously as #288, but with some more.)

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

ipn/local.go

@@ -698,6 +698,7 @@ func (b *LocalBackend) enterState(newState State) {
 	state := b.state
 	prefs := b.prefs
 	notify := b.notify
+	c := b.c
 	b.mu.Unlock()
 
 	if state == newState {
@@ -719,6 +720,9 @@ func (b *LocalBackend) enterState(newState State) {
 		if err != nil {
 			b.logf("Reconfig(down): %v", err)
 		}
+		if c != nil {
+			c.Shutdown()
+		}
 	case Starting, NeedsMachineAuth:
 		b.authReconfig()
 		// Needed so that UpdateEndpoints can run