feature/featuretags: add build tag to remove captive portal detection

This doesn't yet fully pull it out into a feature/captiveportal package.
This is the usual first step, moving the code to its own files within
the same packages.

Updates #17254

Change-Id: Idfaec839debf7c96f51ca6520ce36ccf2f8eec92
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_captiveportal_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_captiveportal
+
+package buildfeatures
+
+// HasCaptivePortal is whether the binary was built with support for modular feature "Captive portal detection".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_captiveportal" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasCaptivePortal = false