* shell: s3.* commands output JSON, connection messages to stderr
All s3.user.* and s3.policy.attach|detach commands now output structured
JSON to stdout instead of human-readable text:
- s3.user.create: {"name","access_key"} (secret key to stderr only)
- s3.user.list: [{name,status,policies,keys}]
- s3.user.show: {name,status,source,account,policies,credentials,...}
- s3.user.delete: {"name"}
- s3.user.enable/disable: {"name","status"}
- s3.policy.attach/detach: {"policy","user"}
Connection startup messages (master/filer) moved to stderr so they
don't pollute structured output when piping.
Closes#8962 (partial — covers merged s3.user/policy commands).
* shell: fix secret leak, duplicate JSON output, and non-interactive prompt
- s3.user.create: only echo secret key to stderr when auto-generated,
never echo caller-supplied secrets
- s3.user.enable/disable: fix duplicate JSON output — remove inner
write in early-return path, keep single write site after gRPC call
- shell_liner: use bufio.Scanner when stdin is not a terminal instead
of liner.Prompt, suppressing the "> " prompt in piped mode
* shell: check scanner error, idempotent enable output, history errors to stderr
- Check scanner.Err() after non-interactive input loop to surface read errors
- s3.user.enable: always emit JSON regardless of current state (idempotent)
- saveHistory: write error messages to stderr instead of stdout
74905c4b5d