New port: dns/flamethrower

Flamethrower is a small, fast, configurable tool for functional
testing, benchmarking, and stress testing DNS servers and networks.
It supports IPv4, IPv6, UDP and TCP, and has a modular system for
generating queries used in the tests.

It was built as an alternative to dnsperf, and many of the command
line options are compatible.

WWW: https://github.com/DNS-OARC/flamethrower

PR: 235521
Submitted by: Leo Vandewoestijne <freebsd@dns.company>