ipn/ipnlocal, feature/posture: pull posture out into a modular feature

Updates #12614

Change-Id: I9d08a1330b9c55e1a23e7979a707e11d8e090d79
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_posture_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_posture
+
+package buildfeatures
+
+// HasPosture is whether the binary was built with support for modular feature "Device posture checking support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_posture" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasPosture = false

feature/buildfeatures/feature_posture_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_posture
+
+package buildfeatures
+
+// HasPosture is whether the binary was built with support for modular feature "Device posture checking support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_posture" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasPosture = true