dns/knot2: update 2.6.8 -> 2.7.3
PR: 232284
Submitted by: Leo Vandewoestijne <freebsd@dns.company> (maintainer)
Relnotes: https://www.knot-dns.cz/2018-08-13-version-269.html
https://www.knot-dns.cz/2018-08-14-version-271.html https://www.knot-dns.cz/2018-08-29-version-272.html https://www.knot-dns.cz/2018-10-11-version-273.html