DHCP: fix chaddr parsing

[gpotter2]

• chaddr should be able to take a MAC address as input and properly pad it
• chaddr should be able to handle RandMAC() or similar VolatileValues

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.54%. Comparing base (1b690bf) to head (94e325a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/dhcp.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4666   +/-   ##
=======================================
  Coverage   81.54%   81.54%           
=======================================
  Files         359      359           
  Lines       86559    86565    +6     
=======================================
+ Hits        70583    70592    +9     
+ Misses      15976    15973    -3     

Files with missing lines	Coverage Δ
scapy/fields.py	92.86% <100.00%> (ø)
scapy/layers/dhcp.py	82.85% <50.00%> (-0.64%)	⬇️

... and 4 files with indirect coverage changes