Identify unused source files via gcov CI job

Once gcov CI job is present (via !3606 (merged)), detailed HTML reports with test code coverage will be generated as artifacts. With their help we should easily identify files, which are compiled-in but their code is likely not being leveraged (such files will have 0% test coverage). Code blocks of dead code might too be identified this way.

Examples include:

lib/dns/portlist.c
lib/isc/bufferlist.c

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