export-tar/import-tar: support for POSIX ACLs

[ThomasWaldmann]

Implemented handling of POSIX access and default ACLs in tar files. New keys, SCHILY.acl.access and SCHILY.acl.default, are used to store these ACLs in the tar PAX headers.

See #2521.

[codecov[bot]]

Codecov Report

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

Project coverage is 82.20%. Comparing base (31b369f) to head (cb3da55). Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8856      +/-   ##
==========================================
- Coverage   82.49%   82.20%   -0.29%     
==========================================
  Files          74       74              
  Lines       13336    13348      +12     
  Branches     1975     1979       +4     
==========================================
- Hits        11001    10973      -28     
- Misses       1675     1706      +31     
- Partials      660      669       +9     

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