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_bsdconst.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build freebsd || openbsd
+
+package routetable
+
+import "golang.org/x/sys/unix"
+
+const (
+	ribType        = unix.NET_RT_DUMP
+	parseType      = unix.NET_RT_IFLIST
+	rmExpectedType = unix.RTM_GET
+
+	// Nothing to skip
+	skipFlags = 0
+)
+
+var flags = map[int]string{
+	unix.RTF_BLACKHOLE: "blackhole",
+	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",
+	unix.RTF_UP:        "up",
+}