[knot-dns] initial integration

[jfoote]

This adds the initial integration for knot-dns. Fuzz targets build/execute as part of knot CI; use corpora from external repos.

Note that the fuzz target builds use the clang6 -fsanitize=fuzzer-no-link flag for compilation and -fsanitize=fuzzer along with -lFuzzingEngine for linking (other sanitizer flags notwithstanding). I didn't find any guidance on the new clang6 flags so I made an educated guess -- if that is incorrect please let me know.

References: initial oss-fuzz project submission, knot-dns merge (gitlab)

[alex]

You generally shouldn't need to specify your own -fsanitize flags, they'll automatically be in the $CFLAGS/$CXXFLAGS env vars.

[jfoote]

That is a good point @alex, thanks; nothing special about the clang6 libfuzzer flags. I'll make another PR to knot-dns to update the build system accordingly and re-open this later.