The device name "tailscale0" will be used for all platforms except for
OpenBSD where "tun" is enforced by the kernel. `CreateTUN()` in
`wireguard-go` will select the next available "tunX" device name on the
OpenBSD system.
Signed-off-by: Martin Baillie <martin@baillie.email>
OpenBSD tunnel names are prefixed with `tun`.
Controlling the port allows for deterministic configuration of firewall
rules (using `pf` in this case).
Signed-off-by: Martin Baillie <martin@baillie.email>