Use pcapng as the name of the file format.

Use pcapng as the name of the file format.

At one point, I remember a discussion resulting in the official name of
the next-generation replacement for pcap format being changed to
"pcapng", with no hyphen.

Make tcpdump reflect that.

While we're at it, uppdate to use "macOS" as the name of Apple's
UNIX-for-Macs, as appropriate (don't use it for versions that were still
called Mac OS X or OS X).