Option helper for AutoConfigure

[candlerb]

This isn't really a fully-fledged pull request: I'm seeking advice on how best to expose the option AutoConfigure from RFC 2563.

What the proposed patch does is implement a Byte option, based on Uint16 from dhcpv4/option_maximum_dhcp_message_size.go

However, other possibilities would be:

1. Implement a Bool option
2. Implement an enumeration which is specific to AutoConfigure - so that for example, the String() method would return "Auto-Configure: DoNotAutoConfigure" or "Auto-Configure: AutoConfigure"
3. Do nothing and require clients to use GenericOption directly. However there could be a small helper for setting this generic option, along the lines of OptClientIdentifier() in dhcpv4/option_misc.go

Based on the registry, some other examples of single-byte DHCP options are:

• 19 Forward On/Off (RFC 2132)
• 20 SrcRte On/Off (RFC 2132)
• 23 Default IP TTL (RFC 2132)
• ... various other obsolete-looking options with length 1
• Weirdly, option 116 (Auto-Config) is described as having a length of N, not 1.

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 73.35%. Comparing base (3c11703) to head (45ea332).

Files	Patch %	Lines
dhcpv4/option_autoconfigure.go	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   73.21%   73.35%   +0.14%     
==========================================
  Files          80       81       +1     
  Lines        5073     5104      +31     
==========================================
+ Hits         3714     3744      +30     
- Misses       1216     1217       +1     
  Partials      143      143              

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[candlerb]

Ready for review

[candlerb]

I have changed the AutoConfigure() helper to return (value, bool) instead of (value, error). I do need this one for coredhcp.

Please could you kick off the CI, and if it's happy, I'm good to merge this now.