feature/clientupdate: move clientupdate to a modular feature, disabled for tsnet

Updates #12614

Change-Id: I5f685dec84a5396b7c2b66f2788ae3d286e1ddc6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_clientupdate_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_clientupdate
+
+package buildfeatures
+
+// HasClientUpdate is whether the binary was built with support for modular feature "Client auto-update support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_clientupdate" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasClientUpdate = false

feature/buildfeatures/feature_clientupdate_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_clientupdate
+
+package buildfeatures
+
+// HasClientUpdate is whether the binary was built with support for modular feature "Client auto-update support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_clientupdate" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasClientUpdate = true