Cleanup getmacbyip and add in4_is*

[gpotter2]

I think in6_is* functions are cool, and it's sad they have no equivalent for IPv4. It also makes the code more readable as those functions are very well documented.

This PR:

• adds in4_is* functions that mimic the IPv6 ones in utils.py
• cleanup getmacbyip. use those functions
• minor cleanups to utils6.py

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.56%. Comparing base (18082f3) to head (044d1b0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4386      +/-   ##
==========================================
- Coverage   81.56%   81.56%   -0.01%     
==========================================
  Files         352      352              
  Lines       83920    83938      +18     
==========================================
+ Hits        68448    68460      +12     
- Misses      15472    15478       +6     

Files	Coverage Δ
scapy/layers/inet6.py	88.46% <100.00%> (+<0.01%)	⬆️
scapy/layers/l2.py	84.32% <100.00%> (ø)
scapy/utils6.py	87.18% <100.00%> (-0.20%)	⬇️
scapy/utils.py	73.12% <96.00%> (+0.26%)	⬆️

... and 5 files with indirect coverage changes

[guedou]

FTR, flake8 complains about an import error