clientupdate: manually restart Windows GUI after update (#9906)

When updating via c2n, `tailscale.exe update` runs from `tailscaled.exe`
which runs as SYSTEM. The MSI installer does not start the GUI when
running as SYSTEM. This results in Tailscale just existing on
auto-update, which is ungood.

Instead, always ask the MSI installer to not launch the GUI (via
`TS_NOLAUNCH` argument) and launch it manually with a token from the
current logged in user. The token code was borrowed from
d9081d6ba2/net/dns/wsl_windows.go (L207-L232)

Also, make some logging changes so that these issues are easier to debug
in the future.

Updates #755

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate_windows.go

@@ -7,13 +7,20 @@
 package clientupdate
 
 import (
+	"os/exec"
+	"os/user"
+	"path/filepath"
+	"syscall"
+
 	"golang.org/x/sys/windows"
+	"tailscale.com/util/winutil"
 	"tailscale.com/util/winutil/authenticode"
 )
 
 func init() {
 	markTempFileFunc = markTempFileWindows
 	verifyAuthenticode = verifyTailscale
+	launchTailscaleAsWinGUIUser = launchTailscaleAsGUIUser
 }
 
 func markTempFileWindows(name string) error {
@@ -26,3 +33,25 @@ const certSubjectTailscale = "Tailscale Inc."
 func verifyTailscale(path string) error {
 	return authenticode.Verify(path, certSubjectTailscale)
 }
+
+func launchTailscaleAsGUIUser(exePath string) error {
+	exePath = filepath.Join(filepath.Dir(exePath), "tailscale-ipn.exe")
+
+	var token windows.Token
+	if u, err := user.Current(); err == nil && u.Name == "SYSTEM" {
+		sessionID := winutil.WTSGetActiveConsoleSessionId()
+		if sessionID != 0xFFFFFFFF {
+			if err := windows.WTSQueryUserToken(sessionID, &token); err != nil {
+				return err
+			}
+			defer token.Close()
+		}
+	}
+
+	cmd := exec.Command(exePath)
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Token:      syscall.Token(token),
+		HideWindow: true,
+	}
+	return cmd.Start()
+}