Add DragonflyBSD detection

[thheinen]

Description

This brings platform detection up to par with Ohai, as DragonFly is the only BSD missing in Train.

Related Issue

As it's a one-liner without side effects, I suppose an own issue is overkill?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New content (non-breaking change)
• Breaking change (a content change which would break existing functionality or processes)

Checklist:

• I have read the CONTRIBUTING document.

[chef-expeditor]

Hello tecracer-theinen! Thanks for the pull request!

Here is what will happen next:

1. Your PR will be reviewed by the maintainers.
2. Possible Outcomes
   a. If everything looks good, one of them will approve it, and your PR will be merged.
   b. The maintainer may request follow-on work (e.g. code fix, linting, etc). We would encourage you to address this work in 2-3 business days to keep the conversation going and to get your contribution in sooner.
   c. Cases exist where a PR is neither aligned to Chef InSpec's product roadmap, or something the team can own or maintain long-term. In these cases, the maintainer will provide justification and close out the PR.

Thank you for contributing!

[Schwad]

Hi @tecracer-theinen - thanks for submitting this. It looks good to me (I know we don't have any declare_bsd/declare_instance tests yet generally).

I will hold on merging on this specifically until we get a second set of core team eyes on this PR and resolve a current release issue we're working through.

[clintoncwolfe]

Thanks!

[tas50]

Thanks @tecracer-theinen