tshttpproxy: support synology proxy configuration

Fixes #4395
Fixes #2605

Signed-off-by: James Tucker <james@tailscale.com>

net/tshttpproxy/tshttpproxy_linux.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2022 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.
+
+//go:build linux
+// +build linux
+
+package tshttpproxy
+
+import (
+	"net/http"
+	"net/url"
+
+	"tailscale.com/version/distro"
+)
+
+func init() {
+	sysProxyFromEnv = linuxSysProxyFromEnv
+}
+
+func linuxSysProxyFromEnv(req *http.Request) (*url.URL, error) {
+	if distro.Get() == distro.Synology {
+		return synologyProxyFromConfigCached(req)
+	}
+	return nil, nil
+}