Prepare to provide OPENSSL_cleanup.

Prepare to provide OPENSSL_cleanup.

OPENSSL_cleanup() cleans up and deallocates memory in use by the library.
There are a couple of use cases for this, primarily related to memory
leak testing. This will not be called automatically in LibreSSL, which
means that OpenSSL's OPENSSL_NO_INIT_ATEXIT is implied. If code wants to
clean up then they need to explicitly call this themselves.

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