New port: dns/knot3-lib

This port allows to install only libknot, which comes along with
Knot DNS (authoritative), but also is a depency of knot-resolver

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

PR: 249364
Submitted by: Leo Vandewoestijne <freebsd@dns.company>
Relnotes: https://www.knot-resolver.cz/2020-09-08-knot-resolver-5.1.3.html