Previously, a pid check was used to prevent open of the tun(4); this works, but may not make the most sense as we don't prevent the owner process from opening the tun device multiple times.

Additionally, nothing prevents a dying tun(4) from being "reopened" in the middle of tun_destroy as soon as the mutex is unlocked, quickly leading to a bad time. Return EBUSY if we're marked for destruction, as well, and the consumer will need to deal with it. The associated character device will be destroyed in short order.