cmd/syncthing: Warn when running as a super user (fixes #4123)

UID 0 on Unixes, SYSTEM SID on Windows.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4148

cmd/syncthing/superuser_unix.go

@@ -0,0 +1,17 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !windows
+
+package main
+
+import (
+	"os"
+)
+
+func isSuperUser() bool {
+	return os.Geteuid() == 0
+}