Add on-the-fly device setup

This adds a new `DeviceStandby` option; when it is disabled (the default), behavior is unchanged. If it is enabled, tinc-up will not be called during tinc initialization, but will instead be deferred until the first node is
reachable, and it will be closed as soon as no nodes are reachable.

Furthermore, on Windows, this also controls when the virtual network interface will have its "cable" "plugged".

This is useful because it means the device won't be set up until we are fairly sure there is something listening on the other side. This is more user-friendly, as one can check on the status of the tinc network connection just by checking the status of the network interface. Besides, it prevents the OS from thinking it is connected to some network when it is in fact completely isolated, a useful behavior especially on Windows.