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).
Remove lots of $Header's and a few $Id's that all belong to the former
CVS repository of tcpdump itself. These keywords have been frozen since
the migration to git in late 2008.
