openbsd: use openssl, remove libressl support, fixes #6474

[ThomasWaldmann]

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[codecov-commenter]

Codecov Report

Merging #6593 (10f3143) into master (3ef355a) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 10f3143 differs from pull request most recent head be9e7d3. Consider uploading reports for the commit be9e7d3 to get more accurate results

@@            Coverage Diff             @@
##           master    #6593      +/-   ##
==========================================
+ Coverage   82.91%   82.94%   +0.02%     
==========================================
  Files          39       39              
  Lines       10672    10669       -3     
  Branches     2095     2093       -2     
==========================================
  Hits         8849     8849              
+ Misses       1309     1306       -3     
  Partials      514      514              

Impacted Files	Coverage Δ
src/borg/archiver.py	78.92% <ø> (+0.10%)	⬆️

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[ThomasWaldmann]

tests on openbsd current in vagrant also worked. thanks @bket!