cmd/tailscale,controlclient,ipnlocal: fix 'up', deflake tests more

The CLI's "up" is kinda chaotic and LocalBackend.Start is kinda
chaotic and they both need to be redone/deleted (respectively), but
this fixes some buggy behavior meanwhile. We were previously calling
StartLoginInteractive (to start the controlclient's RegisterRequest)
redundantly in some cases, causing test flakes depending on timing and
up's weird state machine.

We only need to call StartLoginInteractive in the client if Start itself
doesn't. But Start doesn't tell us that. So cheat a bit and a put the
information about whether there's a current NodeKey in the ipn.Status.
It used to be accessible over LocalAPI via GetPrefs as a private key but
we removed that for security. But a bool is fine.

So then only call StartLoginInteractive if that bool is false and don't
do it in the WatchIPNBus loop.

Fixes #12028
Updates #12042

Change-Id: I0923c3f704a9d6afd825a858eb9a63ca7c1df294
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -790,6 +790,7 @@ func (b *LocalBackend) UpdateStatus(sb *ipnstate.StatusBuilder) {
 			s.ClientVersion = b.lastClientVersion
 		}
 		s.Health = b.health.AppendWarnings(s.Health)
+		s.HaveNodeKey = b.hasNodeKeyLocked()
 
 		// TODO(bradfitz): move this health check into a health.Warnable
 		// and remove from here.
@@ -4605,6 +4606,9 @@ func (b *LocalBackend) resetControlClientLocked() controlclient.Client {
 		return nil
 	}
 
+	b.authURL = ""
+	b.authURLSticky = ""
+
 	// When we clear the control client, stop any outstanding netmap expiry
 	// timer; synthesizing a new netmap while we don't have a control
 	// client will break things.