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_freebsd.go

@@ -20,6 +20,8 @@ import (
 // Work is currently underway for an in-kernel FreeBSD implementation of wireguard
 // https://svnweb.freebsd.org/base?view=revision&revision=357986
 
+const DefaultTunName = "tailscale0"
+
 type freebsdRouter struct {
 	logf    logger.Logf
 	tunname string