ipn, ipnserver, cmd/tailscale: add "server mode" support on Windows

This partially (but not yet fully) migrates Windows to tailscaled's
StateStore storage system.

This adds a new bool Pref, ForceDaemon, defined as:

// ForceDaemon specifies whether a platform that normally
// operates in "client mode" (that is, requires an active user
// logged in with the GUI app running) should keep running after the
// GUI ends and/or the user logs out.
//
// The only current applicable platform is Windows. This
// forced Windows to go into "server mode" where Tailscale is
// running even with no users logged in. This might also be
// used for macOS in the future. This setting has no effect
// for Linux/etc, which always operate in daemon mode.

Then, when ForceDaemon becomes true, we now write use the StateStore
to track which user started it in server mode, and store their prefs
under that key.

The ipnserver validates the connections/identities and informs that
LocalBackend which userid is currently in charge.

The GUI can then enable/disable server mode at runtime, without using
the CLI.

But the "tailscale up" CLI was also fixed, so Windows users can use
authkeys or ACL tags, etc.

Updates #275

cmd/tailscale/cli/down.go

@@ -56,7 +56,6 @@ func runDown(ctx context.Context, args []string) error {
 			}
 			return
 		}
-		log.Printf("Notify: %#v", n)
 	})
 
 	bc.RequestStatus()

cmd/tailscale/cli/up.go

@@ -206,6 +206,8 @@ func runUp(ctx context.Context, args []string) error {
 	prefs.AdvertiseTags = tags
 	prefs.NoSNAT = !upArgs.snat
 	prefs.Hostname = upArgs.hostname
+	prefs.ForceDaemon = (runtime.GOOS == "windows")
+
 	if runtime.GOOS == "linux" {
 		switch upArgs.netfilterMode {
 		case "on":
@@ -229,6 +231,7 @@ func runUp(ctx context.Context, args []string) error {
 	startLoginInteractive := func() { loginOnce.Do(func() { bc.StartLoginInteractive() }) }
 
 	bc.SetPrefs(prefs)
+
 	opts := ipn.Options{
 		StateKey: ipn.GlobalDaemonStateKey,
 		AuthKey:  upArgs.authKey,
@@ -258,6 +261,22 @@ func runUp(ctx context.Context, args []string) error {
 			}
 		},
 	}
+
+	// On Windows, we still run in mostly the "legacy" way that
+	// predated the server's StateStore. That is, we send an empty
+	// StateKey and send the prefs directly. Although the Windows
+	// supports server mode, though, the transition to StateStore
+	// is only half complete. Only server mode uses it, and the
+	// Windows service (~tailscaled) is the one that computes the
+	// StateKey based on the connection idenity. So for now, just
+	// do as the Windows GUI's always done:
+	if runtime.GOOS == "windows" {
+		// The Windows service will set this as needed based
+		// on our connection's identity.
+		opts.StateKey = ""
+		opts.Prefs = prefs
+	}
+
 	// We still have to Start right now because it's the only way to
 	// set up notifications and whatnot. This causes a bunch of churn
 	// every time the CLI touches anything.