netns,wgengine: add OpenBSD support to netns via an rtable

When an exit node has been set and a new default route is added,
create a new rtable in the default rdomain and add the current
default route via its physical interface.  When control() is
requesting a connection not go through the exit-node default route,
we can use the SO_RTABLE socket option to force it through the new
rtable we created.

Updates #17321

Signed-off-by: joshua stein <jcs@jcs.org>

net/netns/netns_default.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !linux && !windows && !darwin
+//go:build !linux && !windows && !darwin && !openbsd
 
 package netns