Add --force-update: immediate signed-update check, stage + service restart

internal/service/service_linux.go

@@ -186,3 +186,16 @@ func Remove() error {
 	}
 	return nil
 }
+
+// RestartIfRunning restarts the systemd unit when it is active (used after
+// a forced update staged a new binary). Reports whether a restart
+// happened. An inactive or missing unit is not an error. Needs root.
+func RestartIfRunning() (bool, error) {
+	if err := exec.Command("systemctl", "is-active", "--quiet", Name+".service").Run(); err != nil {
+		return false, nil // inactive or not installed
+	}
+	if out, err := exec.Command("systemctl", "restart", Name+".service").CombinedOutput(); err != nil {
+		return false, fmt.Errorf("systemctl restart (run as root): %w (%s)", err, out)
+	}
+	return true, nil
+}