Add command-line option to write unprocessed packets to a file

This commit also changes the PcapDumper to automatically flush after
every called to Dump(). This is because pcap_dump has an internal buffer
of some sort that only writes to the file after a set amount of bytes.
When using the new option on a low-traffic network, it might be a while
before you see any packets written since it has to overcome that buffer
limit first.

src/Options.h

@@ -72,6 +72,7 @@ struct Options
 	std::optional<std::string> random_seed_output_file;
 	std::optional<std::string> process_status_file;
 	std::optional<std::string> zeekygen_config_file;
+	std::optional<std::string> unprocessed_output_file;
 
 	std::set<std::string> plugins_to_load;
 	std::vector<std::string> scripts_to_load;