DNS - correct rr type 0 to be in line with RFC

[wesinator]

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

set DNS rr type 0 label to "RESERVED", per latest IANA + RFC
https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-4

see #4416 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.56%. Comparing base (b4bf3d6) to head (bad7ca5).
Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4425      +/-   ##
==========================================
- Coverage   81.57%   81.56%   -0.01%     
==========================================
  Files         352      352              
  Lines       84016    84032      +16     
==========================================
+ Hits        68532    68537       +5     
- Misses      15484    15495      +11     

Files	Coverage Δ
scapy/layers/dns.py	87.63% <ø> (ø)

... and 9 files with indirect coverage changes

[gpotter2]

@guedou I think I'm okay with this PR. You decide though