* sprinkle IPAddress(...).isSet() across our loops to avoid polling on a
stopped interface. status callback and netif_is_up **does not
guarantee and we could use the interface**!
* register *one* status callback per instance, e.g. when begin() is called
multiple times (also notice a subtle issue with schedule function when
instance is delete'ed)
* consistent LwipIntf callback signature. no need for rvalue, just pass
stdfunc as-is and let the compiler figure it out
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