cmd/tailscaled: enable hybrid netstack mode on openbsd too

Apparently OpenBSD can forward packets with manual configuration,

https://github.com/tailscale/tailscale/issues/2498#issuecomment-1114216999

But this makes it work by default. People doing things by hand can
set TS_DEBUG_WRAP_NETSTACK=0 in the environment.

Change-Id: Iee5f32252f83af2baa0ebbe3f20ce9fec5f29e96
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -481,7 +481,7 @@ func shouldWrapNetstack() bool {
 		return true
 	}
 	switch runtime.GOOS {
-	case "windows", "darwin", "freebsd":
+	case "windows", "darwin", "freebsd", "openbsd":
 		// Enable on Windows and tailscaled-on-macOS (this doesn't
 		// affect the GUI clients), and on FreeBSD.
 		return true