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/routetable/routetable_bsd.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build darwin || freebsd
+//go:build darwin || freebsd || openbsd
 
 package routetable
 

net/routetable/routetable_bsdconst.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build freebsd
+//go:build freebsd || openbsd
 
 package routetable
 
@@ -21,6 +21,7 @@ var flags = map[int]string{
 	unix.RTF_BROADCAST: "broadcast",
 	unix.RTF_GATEWAY:   "gateway",
 	unix.RTF_HOST:      "host",
+	unix.RTF_LOCAL:     "local",
 	unix.RTF_MULTICAST: "multicast",
 	unix.RTF_REJECT:    "reject",
 	unix.RTF_STATIC:    "static",

net/routetable/routetable_other.go

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