Knot DNS 1.4.3

Hi,

this merge request adds two new Formulas:

userspace-rcu is requirement for Knot DNS
Knot DNS is high-performance authoritative DNSSEC-enabled DNS server (alternative to Bind)

I hope to not be frowned too much upon since I am upstream for Knot DNS 1.4.3 (well, my team is), but since we maintain packages for most Linux distributions for ourself, we thought it might be good to allow homebrew users to install alternative DNS server and DNS utilities (kdig, khost, knsupdate).

The software is open-source and well-maintained (https://www.ohloh.net/p/knot-dns).

Ondrej