Tun: add support for Darwin utun interface

[0x-0ddc0de]

Fixes #4049

Darwin creates tunnel interfaces differently than BSD, breaking the existing handling which is based on generic BSD tun. This commit adds a new tunnel layer specific to the Darwin utun interface.

This PR picks up the work after the draft PR #4100 got abandoned 2 years ago.

Checklist:

• [x ] If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• [x ] I squashed commits belonging together
• [x ] I added unit tests or explained why they are not relevant
• [x ] 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

TunTapInterface attempts to handle tunnels in Darwin by following the BSD code path. However, this fails for multiple reasons, including the initial rejection of the interface name beginning with "utun" rather than "tun". This PR adds special-case handling for Darwin.

fixes #4049

[codecov]

Codecov Report

❌ Patch coverage is 51.51515% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.73%. Comparing base (d6a25bb) to head (1608390).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/tuntap.py	51.51%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4816      +/-   ##
==========================================
- Coverage   80.98%   80.73%   -0.25%     
==========================================
  Files         365      365              
  Lines       89112    89132      +20     
==========================================
- Hits        72167    71963     -204     
- Misses      16945    17169     +224     

Files with missing lines	Coverage Δ
scapy/layers/tuntap.py	76.47% <51.51%> (-6.36%)	⬇️

... and 20 files with indirect coverage changes

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[0x-0ddc0de]

Hi @micolous,
You were a reviewer on the draft PR for this issue. That draft PR got abandoned and I completed the work in this new PR. Are you available to review this PR? Thanks