pcap_dump_file(3pcap) on macOS

The text f0fc355bcf6b, as macOS 10.14.6, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

get the standard I/O stream for a savefile being written

pcap_dump_file(3pcap) on macOS now