cmd/tailscale: fix up --reset, again

Also fix a somewhat related printing bug in the process where
some paths would print "Success." inconsistently even
when there otherwise was no output (in the EditPrefs path)

Fixes #3830
Updates #3702 (which broke it once while trying to fix it)

Change-Id: Ic51e14526ad75be61ba00084670aa6a98221daa5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/vms/vms_test.go

@@ -376,6 +376,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 	t.Run("login", func(t *testing.T) {
 		runTestCommands(t, timeout, cli, []expect.Batcher{
 			&expect.BSnd{S: fmt.Sprintf("tailscale up --login-server=%s\n", loginServer)},
+			&expect.BSnd{S: "echo Success.\n"},
 			&expect.BExp{R: `Success.`},
 		})
 	})