rec: disallow (by answering Refused) RD=0 by default

[omoerbeek]

Fixes #13386 using the option 2: send Refused before actual resolving is done. Subsequent queries for the same name with RD=0 wil be answered by the packet cache.

Short description

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 7083879960

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• 1 of 9 (11.11%) changed or added relevant lines in 2 files are covered.
• 3858 unchanged lines in 77 files lost coverage.
• Overall coverage increased (+0.2%) to 57.64%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/recursordist/pdns_recursor.cc	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
pdns/backends/gsql/gsqlbackend.hh	1	97.71%
pdns/recursordist/pdns_recursor.cc	1	1.35%
pdns/test-bindparser_cc.cc	1	94.44%
pdns/tsigverifier.cc	1	77.22%
pdns/dbdnsseckeeper.cc	2	85.88%
pdns/dnsdistdist/test-dnsdistluanetwork.cc	2	98.64%
pdns/recursordist/test-negcache_cc.cc	2	98.77%
pdns/recursordist/testrunner.cc	2	89.09%
pdns/test-communicator_hh.cc	2	95.51%
pdns/test-dnsparser_cc.cc	2	99.11%

Totals
Change from base Build 6890076727:	0.2%
Covered Lines:	106651
Relevant Lines:	153851

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💛 - Coveralls

[rgacogne]

PR looks good to me, and the new feature makes sense as well. A regression test checking that RD=0 queries are actually refused might be nice?

[omoerbeek]

Yes, will add test before merge.