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/store.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -33,6 +34,15 @@ const (
 	// node-global state. To keep open the option of having per-user state
 	// later, the global state key doesn't look like a username.
 	GlobalDaemonStateKey = StateKey("_daemon")
+
+	// ServerModeStartKey's value, if non-empty, is the value of a
+	// StateKey containing the prefs to start with which to start the
+	// server.
+	//
+	// For example, the value might be "user-1234", meaning the
+	// the server should start with the Prefs JSON loaded from
+	// StateKey "user-1234".
+	ServerModeStartKey = StateKey("server-mode-start-key")
 )
 
 // StateStore persists state, and produces it back on request.
@@ -132,6 +142,9 @@ func (s *FileStore) ReadState(id StateKey) ([]byte, error) {
 func (s *FileStore) WriteState(id StateKey, bs []byte) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	if bytes.Equal(s.cache[id], bs) {
+		return nil
+	}
 	s.cache[id] = append([]byte(nil), bs...)
 	bs, err := json.MarshalIndent(s.cache, "", "  ")
 	if err != nil {