Add OpenBSD rc.d script.

[skarekrow]

This has a few prerequisites:

• Creating a _cloudflare_ddns user
• Add a cloudflare_ddns section to your /etc/login.conf (for env variables, this is noted in the accompanying README commit

Relies on #733

[skarekrow]

@favonia I tried to keep the style of your README, feel free to edit if you want it laid out differently. I also named the directory in the repo as rc.d, but it could just as easily be init, I'll leave that up to you. I think most non-systemd init systems use rc.d as the naming, but I'm not 100%

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.49%. Comparing base (fc583ad) to head (c7bda51). Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #736   +/-   ##
=======================================
  Coverage   88.49%   88.49%           
=======================================
  Files          48       48           
  Lines        1990     1990           
=======================================
  Hits         1761     1761           
  Misses        208      208           
  Partials       21       21           

Flag	Coverage Δ
unittests	88.49% <ø> (ø)

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[favonia]

@skarekrow Thank you! Could you put the files under contrib/ because, sadly, I will not be able to personally maintain these files? Feel free to update the README and/or rearrange files there.

[skarekrow]

@favonia Whoops didn't even see the contrib directory! I moved the systemd services to have a similar structure and updated the README's for both.