Refactor of stunnel system

[mikenye]

• Improvements due to my growing understanding of Go.
• Integrate new API code into feedproxy.
• Update tests.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.07843% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.17%. Comparing base (59546f6) to head (1b8754f). Report is 1 commits behind head on main.

Files	Patch %	Lines
bordercontrol/lib/listener/listener.go	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #83 +/- ## ========================================== + Coverage 82.87% 83.17% +0.30% ========================================== Files 19 19 Lines 2125 2116 -9 ========================================== - Hits 1761 1760 -1 + Misses 245 239 -6 + Partials 119 117 -2 ```

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