[pdnsutil] dedup in add-record

[miodvallat]

Short description

The discussion of #4727 suggests that adding some smarts to add-record for it not to insert duplicate content would be welcome.

This PR does exactly this, and also tries to make the "Thou shalt not mix CNAME with other records" logic easier to follow.

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)

[coveralls]

Pull Request Test Coverage Report for Build 13411269542

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.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 225 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.001%) to 64.485%

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Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-backend.cc	1	66.19%
pdns/pollmplexer.cc	1	83.66%
modules/lmdbbackend/lmdbbackend.hh	2	90.0%
pdns/misc.cc	2	62.51%
pdns/recursordist/sortlist.cc	2	72.94%
pdns/tcpiohandler.cc	2	68.18%
pdns/recursordist/syncres.cc	3	80.09%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/remote_logger.cc	3	56.95%
pdns/dnsdistdist/dnsdist-tcp.cc	8	75.76%

Totals
Change from base Build 13368084756:	-0.001%
Covered Lines:	127619
Relevant Lines:	166885

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💛 - Coveralls

[Habbie]

unfinished review, no negative comments so far