pcap listener doesn't shutdown on ctrl-c

[msiodelski]

As discussed in the following thread: the-tcpdump-group/libpcap#572, on modern Linux distributions where TPACKET_V3 is used, setting the timeout value on the capture does not break listening after the timeout before receiving the first packet. As a result, the current endure code blocks infinitely without checking if the dispatcher should shutdown. Killing the process is the only way to exit. It can be mitigated be performing an active check if the shutdown is progressing and simply exit if it doesn't.

[msiodelski]

Fixed with #6