feature/linkspeed: move cosmetic tstun netlink code out to modular feature

Part of making all netlink monitoring code optional.

Updates #17311 (how I got started down this path)
Updates #12614

Change-Id: Ic80d8a7a44dc261c4b8678b3c2241c3b3778370d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_linkspeed_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_linkspeed
+
+package buildfeatures
+
+// HasLinkSpeed is whether the binary was built with support for modular feature "Set link speed on TUN device for better OS integration (Linux only)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_linkspeed" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasLinkSpeed = false

feature/buildfeatures/feature_linkspeed_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_linkspeed
+
+package buildfeatures
+
+// HasLinkSpeed is whether the binary was built with support for modular feature "Set link speed on TUN device for better OS integration (Linux only)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_linkspeed" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasLinkSpeed = true