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

ipn/local.go

@@ -66,7 +66,8 @@ type LocalBackend struct {
 	mu             sync.Mutex
 	notify         func(Notify)
 	c              *controlclient.Client
-	stateKey       StateKey
+	stateKey       StateKey // computed in part from user-provided value
+	userID         string   // current controlling user ID (for Windows, primarily)
 	prefs          *Prefs
 	machinePrivKey wgcfg.PrivateKey
 	state          State
@@ -786,6 +787,15 @@ func (b *LocalBackend) State() State {
 	return b.state
 }
 
+// RunningAndDaemonForced reports whether the backend is currently
+// running and the preferences say that Tailscale should run in
+// "server mode" (ForceDaemon).
+func (b *LocalBackend) RunningAndDaemonForced() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.state == Running && b.prefs != nil && b.prefs.ForceDaemon
+}
+
 // getEngineStatus returns a copy of b.engineStatus.
 //
 // TODO(bradfitz): remove this and use Status() throughout.
@@ -899,6 +909,12 @@ func (b *LocalBackend) shieldsAreUp() bool {
 	return b.prefs.ShieldsUp
 }
 
+func (b *LocalBackend) SetCurrentUserID(uid string) {
+	b.mu.Lock()
+	b.userID = uid
+	b.mu.Unlock()
+}
+
 func (b *LocalBackend) SetWantRunning(wantRunning bool) {
 	b.mu.Lock()
 	new := b.prefs.Clone()
@@ -935,6 +951,7 @@ func (b *LocalBackend) SetPrefs(new *Prefs) {
 	applyPrefsToHostinfo(newHi, new)
 	b.hostinfo = newHi
 	hostInfoChanged := !oldHi.Equal(newHi)
+	userID := b.userID
 
 	b.mu.Unlock()
 
@@ -943,6 +960,26 @@ func (b *LocalBackend) SetPrefs(new *Prefs) {
 			b.logf("Failed to save new controlclient state: %v", err)
 		}
 	}
+	if userID != "" { // e.g. on Windows
+		if new.ForceDaemon {
+			stateKey := StateKey("user-" + userID)
+			if err := b.store.WriteState(ServerModeStartKey, []byte(stateKey)); err != nil {
+				b.logf("WriteState error: %v", err)
+			}
+			// It's important we do this here too, even if it looks
+			// redundant with the one in the 'if stateKey != ""'
+			// check block above. That one won't fire in the case
+			// where the Windows client started up in client mode.
+			// This happens when we transition into server mode:
+			if err := b.store.WriteState(stateKey, new.ToBytes()); err != nil {
+				b.logf("WriteState error: %v", err)
+			}
+		} else {
+			if err := b.store.WriteState(ServerModeStartKey, nil); err != nil {
+				b.logf("WriteState error: %v", err)
+			}
+		}
+	}
 
 	// [GRINDER STATS LINE] - please don't remove (used for log parsing)
 	b.logf("SetPrefs: %v", new.Pretty())