* S3: map canned ACL to file permissions and add configurable default file mode
S3 uploads were hardcoded to 0660 regardless of ACL headers. Now the
X-Amz-Acl header maps to Unix file permissions per-object:
- public-read, authenticated-read, bucket-owner-read → 0644
- public-read-write → 0666
- private, bucket-owner-full-control → 0660
Also adds -defaultFileMode / -s3.defaultFileMode flag to set a
server-wide default when no ACL header is present.
Closes#8874
* Address review feedback for S3 file mode feature
- Extract hardcoded 0660 to defaultFileMode constant
- Change parseDefaultFileMode to return error instead of calling Fatalf
- Add -s3.defaultFileMode flag to filer.go and mini.go (was missing)
- Add doc comment to S3Options about updating all four flag sites
- Add TestResolveFileMode with 10 test cases covering ACL mapping,
server default, and priority ordering
efbed39e25