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>

ipn/ipnlocal/local.go

@@ -5601,7 +5601,7 @@ func (b *LocalBackend) routerConfigLocked(cfg *wgcfg.Config, prefs ipn.PrefsView
 			b.logf("failed to discover interface ips: %v", err)
 		}
 		switch runtime.GOOS {
-		case "linux", "windows", "darwin", "ios", "android":
+		case "linux", "windows", "darwin", "ios", "android", "openbsd":
 			rs.LocalRoutes = internalIPs // unconditionally allow access to guest VM networks
 			if prefs.ExitNodeAllowLANAccess() {
 				rs.LocalRoutes = append(rs.LocalRoutes, externalIPs...)