Feature: nclient4 BSD/macOS support

[vista-]

This PR adds back BSD/macOS support for nclient4, which was removed when the library switch from using the raw package for sending/receiving broadcast UDP packets, instead using the packet library (which only supports Linux).

The PR also solves the build-scoping issue described in #526

Due to lack of support for raw datagram sockets in BSD, additional functionality had to be added for constructing the raw Ethernet frame. VLAN tag support has also been added to the BSD/macOS-related nclient4 code, which can be utilised to send/receive tagged frames. This can be useful for testing IPoE-based access networks.

Tests have also been added for the Ethernet frame and VLAN tag processing, based on the gopacket library.

There are two avenues of improving the code in the PR in the future (but I would prefer to avoid premature optimisation):

• Create a byte array with the byte representation of the VLAN stack + EtherType field in it during initialisation, in NewRawUDPConn. This can be inserted at the EtherType offset of the Ethernet frame when sending, and performing a simple byte comparison at the EtherType offset when receiving frames.
• Use gopacket for constructing the frame from scratch instead of doing it byte by byte.

[codecov]

Codecov Report

Attention: Patch coverage is 80.23952% with 33 lines in your changes missing coverage. Please review.

Project coverage is 73.65%. Comparing base (bf3278a) to head (4fa4462).

❗ Current head 4fa4462 differs from pull request most recent head 64fb84e

Please upload reports for the commit 64fb84e to get more accurate results.

Files	Patch %	Lines
dhcpv4/nclient4/udp.go	56.09%	12 Missing and 6 partials ⚠️
dhcpv4/nclient4/conn_linux.go	64.28%	13 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
- Coverage   73.77%   73.65%   -0.13%     
==========================================
  Files          81       83       +2     
  Lines        5122     5197      +75     
==========================================
+ Hits         3779     3828      +49     
- Misses       1200     1225      +25     
- Partials      143      144       +1     

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[vista-]

Force pushed commits with sign-offs.

[vista-]

Ping! :)