Make CBC decoding constant time.
This patch makes the decoding of SSLv3 and TLS CBC records constant time. Without this, a timing side-channel can be used to build a padding oracle and mount Vaudenay's attack. This patch also disables the stitched AESNI+SHA mode pending a similar fix to that code. In order to be easy to backport, this change is implemented in ssl/, rather than as a generic AEAD mode. In the future this should be changed around so that HMAC isn't in ssl/, but crypto/ as FIPS expects.
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