New port: dns/knot3

Knot DNS is a high-performance authoritative-only DNS server which
supports all key features of the domain name system including zone
AXFR and IXFR, DDNS and DNSSEC. Its key features:

• Open source
• High-performance, multi-threaded, and mostly lock-free implementation which scales well on SMPs
• Object-oriented design
• Support for all important DNS protocols:
  • Full and incremental zone transfers
  • EDNS0 and DNSSEC extensions, including NSEC3
  • NSID

WWW: https://www.knot-dns.cz/

PR: 249363
Submitted by: Leo Vandewoestijne <freebsd@dns.company>
Relnotes: https://www.knot-dns.cz/2020-09-09-version-300.html