Follow-up to a237043 addressing three security/correctness re-review findings.
(1) MEDIUM: a server-contacting --dry-run with --compare-dest/--copy-dest/
--link-dest still read and hashed the basis file and compared it with the
client-supplied digest, a 1-bit content oracle. basis_match_find() gains a
hash_content parameter; the dry-run shortcut passes false and returns no
match without touching basis bytes, so an otherwise-matching entry is
reported as would-transfer. The real (non-dry-run) path is unchanged.
(2) LOW: xattr_capture_path() hardcoded preserve_acls=true, so the receiver's
hard-link copy fallback re-applied system.posix_acl_* even when -A was not
negotiated. The function now takes preserve_acls and members.* is
unaffected; scanner and receiver callers thread the negotiated flag.
(3) INFO: the --fsync --link-dest temp reopen now uses O_NONBLOCK and treats
a raced-in FIFO's ENXIO as a benign fsync-skip instead of blocking.
Tests: dry-run + basis unit test (asserts would-transfer, no content read) and
integration test; xattr capture ACL-filter test. Verified strict build, ASan,
clang-format, cppcheck, and the CI integration subset.
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