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>