pcap_dump_ftell(3pcap) on macOS

The text 220ac3f9f963, as macOS 10.14.6, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

get the current file offset for a savefile being written

pcap_dump_ftell(3pcap) on macOS now