Minor improvements to Kerberos, IGMP and FWDM

[gpotter2]

• minor improvements to forwarding machine
• IGMP: obvious typo (tox/tos), implement v3 leave
• support password policy errors in kpasswd

[codecov]

Codecov Report

❌ Patch coverage is 34.78261% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.33%. Comparing base (14cc726) to head (1e01c62).

Files with missing lines	Patch %	Lines
scapy/layers/igmp.py	22.22%	7 Missing ⚠️
scapy/fwdmachine.py	0.00%	4 Missing ⚠️
scapy/layers/kerberos.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5019      +/-   ##
==========================================
- Coverage   80.34%   80.33%   -0.01%     
==========================================
  Files         386      386              
  Lines       96012    96031      +19     
==========================================
+ Hits        77137    77150      +13     
- Misses      18875    18881       +6     

Files with missing lines	Coverage Δ
scapy/fwdmachine.py	0.00% <0.00%> (ø)
scapy/layers/kerberos.py	57.56% <60.00%> (+0.01%)	⬆️
scapy/layers/igmp.py	80.82% <22.22%> (-2.60%)	⬇️

... and 5 files with indirect coverage changes

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