Add support for old Info-ZIP extra block for Unix

[bastianeicher]

SharpZipLib currently supports reading timestamps from the "Extended Timestamp Extra Field" (0x5455).

This PR adds support for the older (but still often seen in new .zip files) "Info-ZIP Unix Extra Field (type 1)" (0x5855).

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[codecov[bot]]

Codecov Report

Merging #660 (cef5453) into master (cd5310f) will decrease coverage by 0.24%. The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
- Coverage   73.28%   73.04%   -0.25%     
==========================================
  Files          68       68              
  Lines        8304     8339      +35     
==========================================
+ Hits         6086     6091       +5     
- Misses       2218     2248      +30     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Zip/ZipExtraData.cs	69.04% <9.37%> (-7.29%)	:arrow_down:
src/ICSharpCode.SharpZipLib/Zip/ZipEntry.cs	90.59% <66.66%> (-0.32%)	:arrow_down:

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