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 10942983544

Details

• 28 of 30 (93.33%) changed or added relevant lines in 2 files are covered.
• 24 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.03%) to 64.687%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/recursordist/reczones.cc	22	24	91.67%

Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-crypto.cc	1	76.01%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
modules/gpgsqlbackend/spgsql.cc	3	68.18%
pdns/recursordist/pdns_recursor.cc	3	72.67%
pdns/recursordist/settings/cxxsettings-generated.cc	4	91.61%
modules/godbcbackend/sodbc.cc	4	70.8%
pdns/recursordist/test-syncres_cc1.cc	6	89.59%

Totals
Change from base Build 10939216509:	0.03%
Covered Lines:	124839
Relevant Lines:	162255

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💛 - Coveralls