feature/sdnotify: move util/systemd to a modular feature

Updates #12614

Change-Id: I08e714c83b455df7f538cc99cafe940db936b480
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_sdnotify_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_sdnotify
+
+package buildfeatures
+
+// HasSDNotify is whether the binary was built with support for modular feature "systemd notification support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_sdnotify" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasSDNotify = false

feature/buildfeatures/feature_sdnotify_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_sdnotify
+
+package buildfeatures
+
+// HasSDNotify is whether the binary was built with support for modular feature "systemd notification support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_sdnotify" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasSDNotify = true