xattr: document fakeroot xattr as Linux-only, add missing fakeroot skip on FreeBSD, fixes #9394

[mr-raj12]

Summary

• Resolved open TODOs in src/borg/xattr.py (lines 22-23) that asked whether fakeroot supports xattrs on non-Linux platforms
• Added missing skipif_fakeroot_detected to freebsd_test.py for consistency with linux_test.py and darwin_test.py
• Added test verifying XATTR_FAKEROOT is False on non-Linux and without fakeroot active

Fixes #9394

Background

Investigation confirmed fakeroot xattr support is Linux-only:

• FreeBSD/NetBSD: fakeroot only wraps the Linux-style setxattr/getxattr API, but these platforms use the extattr_* API instead — fakeroot never intercepts xattr calls there
• macOS: fakeroot explicitly #undefs all xattr wrappers due to prototype incompatibilities with the Linux versions
• Vagrantfile confirms: FreeBSD comments "fakeroot causes lots of troubles", NetBSD/OpenBSD have "no fakeroot"

Changes

• src/borg/xattr.py — Replace TODO comments with documentation explaining why detection is Linux-only
• src/borg/testsuite/platform/freebsd_test.py — Add skipif_fakeroot_detected to pytestmark (was missing, unlike linux_test.py and darwin_test.py)
• src/borg/testsuite/xattr_test.py — Add test_xattr_fakeroot_flag() test

Test plan

• CI passes on all platforms
• test_xattr_fakeroot_flag runs and passes
• FreeBSD ACL tests still run normally (skip only triggers under fakeroot)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.43%. Comparing base (77a5f43) to head (d1a10dd).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9396   +/-   ##
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  Coverage   76.43%   76.43%           
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  Files          85       85           
  Lines       14800    14800           
  Branches     2212     2212           
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  Hits        11313    11313           
  Misses       2809     2809           
  Partials      678      678           

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