sslv2: implement stricter 'security' (yeah, we know)

[gpotter2]

There's something poetic about improving the security of our SSLv2 stack.

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.87%. Comparing base (22d3930) to head (a113730).

Files with missing lines	Patch %	Lines
scapy/layers/tls/automaton_cli.py	50.00%	3 Missing ⚠️
scapy/layers/tls/automaton_srv.py	25.00%	3 Missing ⚠️
scapy/layers/tls/record_sslv2.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5197      +/-   ##
==========================================
- Coverage   80.87%   80.87%   -0.01%     
==========================================
  Files         393      393              
  Lines       98082    98097      +15     
==========================================
+ Hits        79327    79336       +9     
- Misses      18755    18761       +6     

Files with missing lines	Coverage Δ
scapy/layers/tls/handshake_sslv2.py	93.58% <100.00%> (+1.60%)	⬆️
scapy/layers/tls/session.py	86.03% <100.00%> (+0.01%)	⬆️
scapy/layers/tls/automaton_cli.py	86.98% <50.00%> (-0.26%)	⬇️
scapy/layers/tls/automaton_srv.py	73.06% <25.00%> (-0.21%)	⬇️
scapy/layers/tls/record_sslv2.py	89.56% <25.00%> (-0.95%)	⬇️

... and 3 files with indirect coverage changes

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