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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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.