feat: un-hardcode SYST response

[rwakulszowa]

What do these changes do?

Un-hardcode SYST response
It should identify the underlying OS. Allow the caller to specify the value.

Are there changes in behavior for the user?

The SYST response is no longer hardcoded to UNIX

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.94%. Comparing base (4552c2d) to head (efa36b7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #208   +/-   ##
=======================================
  Coverage   97.94%   97.94%           
=======================================
  Files           6        6           
  Lines        2094     2095    +1     
=======================================
+ Hits         2051     2052    +1     
  Misses         43       43           

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[pohmelie]

I think I will merge this, but what is the reason for this? Anyone uses SYST at all? I mean... the value of SYST response

[rwakulszowa]

Whether anyone actually cares about the result in practice - I have no idea. The only reason I implemented this was because it's a well known (albeit not too useful) command and someone might wanna use it, so I figured it's better to have it configurable.