wgengine: define default tunname for each platform

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>

wgengine/router_linux.go

@@ -22,6 +22,8 @@ import (
 	"tailscale.com/types/logger"
 )
 
+const DefaultTunName = "tailscale0"
+
 type linuxRouter struct {
 	logf    func(fmt string, args ...interface{})
 	tunname string