The chief use case for this is when `sockfd` was allocated in a different network namespace and passed to the current process over a UNIX domain socket or similar. It allows the current process to use netfilterqueue to manage traffic in a different network namespace.
Propagate exceptions raised by the user's packet callback -- fixes#31, #50
Warn about exceptions raised by the packet callback during queue unbinding
Raise an error if a packet verdict is set after its parent queue is closed
set_payload() now affects the result of later get_payload() -- fixes#30
Handle signals received when run() is blocked in recv() -- fixes#65