tcpdump: Update to 4.99.6

[jrm]

Summary

Changes: https://github.com/the-tcpdump-group/tcpdump/blob/tcpdump-4.99/CHANGES
Obtained from: https://www.tcpdump.org/release/tcpdump-4.99.6.tar.xz
Sponsored by: The FreeBSD Foundation

[jrm]

This update was straightforward and only required manual changes to:

• usr.sbin/tcpdump/tcpdump/config
• usr.sbin/tcpdump/tcpdump/Makefile
• contrib/tcpdump/print-pflog.c

[jrm]

Committed in e6083790f217ba7f89cd2957922bd45e35466359.