Restrict a port to sending ARP/IPv6 ND broadcasts only.

[anarkiwi]

Fixes #1454.

[codecov]

Codecov Report

Merging #3238 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3238      +/-   ##
==========================================
+ Coverage   93.03%   93.05%   +0.02%     
==========================================
  Files          42       42              
  Lines        7289     7314      +25     
==========================================
+ Hits         6781     6806      +25     
  Misses        508      508

Impacted Files	Coverage Δ
faucet/vlan.py	96.9% <100%> (+0.02%)	⬆️
faucet/port.py	96.77% <100%> (+0.02%)	⬆️
faucet/dp.py	96.55% <100%> (+0.03%)	⬆️
faucet/valve_flood.py	91.71% <100%> (+0.33%)	⬆️
faucet/valve.py	94.47% <100%> (+0.01%)	⬆️

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