dns: fix root zone dnssec

[pinheadmz]

Closes #565

Based on james-stevens@18cdabc

TODO:

• Maybe also stop serving TXT (Should HNS root nameserver serve TXT records? #565) (06cf026)

Changes:

• Root always signs DS, never signs NS, and only signs TXT if NS is not present
• Root always sets aa (authority) bit for DS, never for NS, and for TXT only if NS is not present
• Root doesn't answer a query for TXT if NS is present (sends referral instead)
• Root sets authority bit for NXDOMAIN

[coveralls]

Pull Request Test Coverage Report for Build 600554084

• 7 of 9 (77.78%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-8.2%) to 59.379%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/dns/server.js	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
lib/dns/resource.js	1	80.04%
lib/protocol/consensus.js	1	82.79%

Totals
Change from base Build 541449016:	-8.2%
Covered Lines:	19511
Relevant Lines:	30584

---

💛 - Coveralls

[tuxcanfly]

Tested. I checked against the linked issue and rationale seems to be to avoid conflict by preferring NS over TXT from the root zone. It maybe a good idea to update the PR with the conclusion from the linked issue and rationale for making this change.