* s3: allow POST Object uploads when auth is off
With no identities configured every other handler skips authentication,
but PostPolicyBucketHandler still verified the form signature, so a
browser form upload failed with InvalidAccessKeyId (or a malformed
credential error without signature fields). Skip the signature check
when auth is off; the policy conditions are still enforced.
* s3: update AuthPostPolicy comment for the auth-off path
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Co-authored-by: Chris Lu <chris.lu@gmail.com>