pcap_set_snaplen(3pcap) on macOS

The text dbfa3e3b02f4, as macOS 10.14.6, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

set the snapshot length for a not-yet-activated capture handle

pcap_set_snaplen(3pcap) on macOS now