s3: support x-amz-meta-mode for symlinks (s3fs-fuse interop)

[mkviatkovskii]

Goal

Make the s3 gateway compatible with s3fs-fuse's symlink convention. Without this, mounting a TernFS bucket via s3fs flattens every ln -s into a regular file containing the target path as data.

Changes

• On PUT, read x-amz-meta-mode. If S_IFLNK is set, create a msgs.SYMLINK inode (TernFS already stores symlink targets as the inode's body — same as ternfuse).
• On GET / HEAD / ?attributes, emit x-amz-meta-mode so s3fs reconstructs the symlink on read.
• Header is parsed base-0 to match s3fs's strtoll(_, _, 0) (decimal, octal, hex all accepted). Missing/garbage = regular file.
• Unit tests in go/s3/s3_test.go for the parser/emitter