feature/routecheck: add ts_omit_routecheck feature flag (#19638)

RouteCheck, which checks that overlapping routers are reachable, is
enabled by default for both tailscaled and tsnet.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

feature/buildfeatures/feature_routecheck_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_routecheck
+
+package buildfeatures
+
+// HasRouteCheck is whether the binary was built with support for modular feature "Support checking the reachability of overlapping routers, for choosing between multiple network paths to the same IP address".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_routecheck" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRouteCheck = false

feature/buildfeatures/feature_routecheck_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_routecheck
+
+package buildfeatures
+
+// HasRouteCheck is whether the binary was built with support for modular feature "Support checking the reachability of overlapping routers, for choosing between multiple network paths to the same IP address".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_routecheck" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRouteCheck = true