illumos/Solaris: do not open device nodes to get their xattrs

[ThomasWaldmann]

On illumos/Solaris, the extended attributes of a file live in a hidden attribute directory, which can only be reached via an fd of the file itself (O_XATTR, see fsattr(7)). So _open_attrdir() opened the file first — for a block or character device node, that runs the open routine of the device driver, which might:

• fail: ENXIO if there is no such device,
• block,
• or have side effects (e.g. a tape rewinding on close).

illumos has no O_PATH to avoid this, so device nodes are now treated as not supporting extended attributes: _open_attrdir() checks the file type first and raises ENOTSUP for block/char devices without opening them, which xattr.get_all() already handles as "no xattrs".

Found when running the test suite as root on OmniOS r151058: the archiver tests create input/bdev and input/cdev with device numbers that have no driver, so borg create got ENXIO from the xattr code, retried each of them for ~15s and then finished with rc 104 (10 failures in create_cmd_test.py: test_basic_functionality, test_create_tags, test_create_archivename_with_placeholder, test_common_options, test_log_json, each for archiver and remote_archiver).

Testing (OmniOS r151058, Python 3.13, as root):

• New test_device_node_is_not_opened (block + char device) fails with errno 6 (ENXIO) without the fix, passes with it.
• Full test suite with the fix: 3264 passed, 1040 skipped, 0 failed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.71%. Comparing base (4897f70) to head (b03b096).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/platform/solaris.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10468      +/-   ##
==========================================
- Coverage   88.72%   88.71%   -0.01%     
==========================================
  Files         103      103              
  Lines       19388    19392       +4     
  Branches     3023     3024       +1     
==========================================
+ Hits        17202    17204       +2     
  Misses       1518     1518              
- Partials      668      670       +2     

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