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

feature/buildfeatures/feature_captiveportal_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_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 = true

feature/featuretags/featuretags.go

@@ -93,6 +93,7 @@ var Features = map[FeatureTag]FeatureMeta{
 	"acme":          {"ACME", "ACME TLS certificate management", nil},
 	"aws":           {"AWS", "AWS integration", nil},
 	"bird":          {"Bird", "Bird BGP integration", nil},
+	"captiveportal": {"CaptivePortal", "Captive portal detection", nil},
 	"capture":       {"Capture", "Packet capture", nil},
 	"cli":           {"CLI", "embed the CLI into the tailscaled binary", nil},
 	"completion":    {"Completion", "CLI shell completion", nil},