wgengine, wgengine/router, cmd/tailscale: force netfilter mode off on Synology

For now. Get it working again so it's not stuck on 0.98.

Subnet relay can come later.

Updates #451

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/distro/distro.go

@@ -0,0 +1,40 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package distro reports which distro we're running on.
+package distro
+
+import (
+	"os"
+	"runtime"
+)
+
+type Distro string
+
+const (
+	Debian   = Distro("debian")
+	Arch     = Distro("arch")
+	Synology = Distro("synology")
+)
+
+// Get returns the current distro, or the empty string if unknown.
+func Get() Distro {
+	if runtime.GOOS == "linux" {
+		return linuxDistro()
+	}
+	return ""
+}
+
+func linuxDistro() Distro {
+	if fi, err := os.Stat("/usr/syno"); err == nil && fi.IsDir() {
+		return Synology
+	}
+	if _, err := os.Stat("/etc/debian_version"); err == nil {
+		return Debian
+	}
+	if _, err := os.Stat("/etc/arch-release"); err == nil {
+		return Arch
+	}
+	return ""
+}