S3: Fix use of HMAC_CTX

[dthain]

s3_file_io.c used an older code pattern of allocating HMAC_CTX on the stack,
and the current practice in SSL is to instead use HMAC_CTX_new/free.

Note that this module has a number of other code smells that I am not cleaning up at this moment.
Also, it was not compiled in CI due to the lack of lib curl.

Proposed changes

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Additional comments

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[dthain]

@btovar any objections?