file flags: fix generic set_flags, keep unprivileged flags on EPERM, warn on failure

[ThomasWaldmann]

Fixes #1345 (leftovers from the #9039 work).

• platform.base.set_flags (used on NetBSD and generic POSIX) now also does the masked read-modify-write cycle instead of setting the archived value as-is. Before, a foreign flag bit made the whole lchflags call fail (losing all flags for that item) and a succeeding call clobbered flags that are not settable from userspace. The flag masks are defined once in platform.base now and shared by the freebsd/darwin implementations.
• EPERM retry: if setting flags fails with EPERM (super-user-only SF_* flags while not running as root, or immutable/append-only on Linux without CAP_LINUX_IMMUTABLE), retry influencing only the unprivileged flags, so e.g. the nodump flag still gets restored by an unprivileged borg extract.
• extract: emit a warning and set the warning exit code if the flags of an item cannot be set (was: silently ignored), consistent with the xattr error handling.
• macOS set_flags: tolerate EOPNOTSUPP like the linux/freebsd implementations already do.
• linux: remove the unused FS_COMPR_FL declaration.
• new tests for the base implementation (faked lstat/lchflags, so they run on all platforms) and for the extract warning; docs section describing cross-platform behavior of file flags.

This answers the two questions from #1345: the flag sets do differ per platform (borg archives BSD-style values, translating on Linux), and extracting an item with flags unsupported on the destination platform drops only those flags — per-flag via the masks, and now also per-flag for flags that merely lack privileges. Only when flags cannot be set at all are the item's flags skipped, with a warning now.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.05%. Comparing base (e15404a) to head (f69e674).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10104      +/-   ##
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+ Coverage   87.01%   87.05%   +0.04%     
==========================================
  Files          99      100       +1     
  Lines       17615    17684      +69     
  Branches     2664     2673       +9     
==========================================
+ Hits        15327    15395      +68     
  Misses       1593     1593              
- Partials      695      696       +1     

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