illumos/Solaris: add xattr support, see #1337

[ThomasWaldmann]

Add xattr support for illumos / Solaris derivatives, see #1337.

On these platforms, the extended attributes of a file are regular files inside a hidden attribute directory attached to that file, accessed by giving O_XATTR to open(2)/openat(2), see fsattr(7). Thus this can be implemented in pure Python (new src/borg/platform/solaris.py, no C extension): xattr names/values map to the names/contents of the files in the attribute directory.

• listxattr: list the attribute directory, hiding the SUNWattr_* "extended system attributes" the OS maintains in there.
• getxattr: read the attribute file (missing attribute file → ENOATTR); values are limited to 16 MiB, like on the other platforms.
• setxattr: create/overwrite the attribute file (refusing names unusable as file names, and SUNWattr_*).
• symlinks cannot have extended attributes on these platforms: listxattr returns an empty result for them, and the set-xattr-on-symlink parts of the xattr tests are now gated accordingly.
• filesystems without xattr support (e.g. tmpfs): ENOTSUP.

With this, xattr.is_enabled() returns True on ZFS, so the xattr tests and the archiver tests' xattr comparisons now actually run in the omniOS CI job (its TMPDIR is ZFS-backed). The job now also probes/logs xattr support on TMPDIR via runat(1). Docs: platform support matrix updated.

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[codecov]

Codecov Report

❌ Patch coverage is 85.91549% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (b893d49) to head (64d622c).
⚠️ Report is 14 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/platform/solaris.py	83.33%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10102      +/-   ##
==========================================
+ Coverage   87.01%   87.02%   +0.01%     
==========================================
  Files          99      101       +2     
  Lines       17615    17730     +115     
  Branches     2664     2678      +14     
==========================================
+ Hits        15327    15429     +102     
- Misses       1593     1599       +6     
- Partials      695      702       +7     

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