Improves pcap error and generic signal handlers.
Adds the -O3 to non-DEVBUILDs.
Splits pcap functionality into `netcapture.{c,h}`, and adds `util.{c,h}`.
Adds Valgrind setup TODO.
Adds more helpful termination logging messages.
Changes netcapture() to take a pcap_t** instead of returning a pcap_t*.

This ensures that the pcap_t* has pcap_close called on it when the program is terminated by a signal.