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/zeek-setup.cc

@@ -708,7 +708,7 @@ SetupResult setup(int argc, char** argv, Options* zopts)
 			exit(success ? 0 : 1);
 			}
 
-		packet_mgr->InitPostScript();
+		packet_mgr->InitPostScript(options.unprocessed_output_file.value_or(""));
 		analyzer_mgr->InitPostScript();
 		file_mgr->InitPostScript();
 		dns_mgr->InitPostScript();