rec: implement rfc6303 special zones (mostly v6 reverse mappings)

[omoerbeek]

Short description

Guarded by a recursor.serve_rfc6303 setting, default true.

Requested by @neilcook

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 11213595982

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 27 of 33 (81.82%) changed or added relevant lines in 2 files are covered.
• 5531 unchanged lines in 74 files lost coverage.
• Overall coverage increased (+0.002%) to 64.724%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/recursordist/reczones.cc	21	27	77.78%

Files with Coverage Reduction	New Missed Lines	%
pdns/webserver.hh	1	69.16%
pdns/recursordist/ext/protozero/include/protozero/config.hpp	1	0.0%
pdns/recursordist/sortlist.hh	1	75.0%
pdns/iputils.hh	1	78.25%
pdns/dnstap.cc	2	70.97%
ext/probds/murmur3.cc	2	94.12%
pdns/sstuff.hh	2	56.83%
pdns/dnsname.hh	2	91.84%
pdns/recursordist/secpoll-recursor.cc	2	51.02%
pdns/rcpgenerator.cc	2	90.05%

Totals
Change from base Build 11213210825:	0.002%
Covered Lines:	125039
Relevant Lines:	162466

---

💛 - Coveralls

[rgacogne]

Are we okay with what happens if serve-rfc6303 is enabled but serve-rfc1918 is not?

[omoerbeek]

I'll look into that. Easiest thing would be to forbid that.

[mnordhoff]

Updates #8679. :D