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_openbsd.go

@@ -25,6 +25,8 @@ import (
 // There is an experimental kernel version in the works for OpenBSD:
 // https://git.zx2c4.com/wireguard-openbsd.
 
+const DefaultTunName = "tun"
+
 type openbsdRouter struct {
 	logf    logger.Logf
 	tunname string