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/routecheck/routecheck.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package routecheck registers support for RouteCheck,
+// which checks the reachability of overlapping routers.
+//
+// When there are multiple network paths to an IP address, it is being routed by
+// overlapping routers. The client uses reachability to pick between those
+// paths: either sticking with an active WireGuard session or choosing from the
+// peers that it has determined it can reach. It doesn’t need reachability for
+// IP addresses that have only one network path, since it can naively attempt to
+// establish a WireGuard session.
+package routecheck
+
+func init() {
+	// TODO(sfllaw): Initialize the new routecheck package.
+}