Handle OpenSSL 1.1.x.

Handle OpenSSL 1.1.x.

In 1.1.x, EVP_CIPHER_CTX is an opaque structure, so we can't declare it
on the stack.

Instead, if we don't have EVP_CIPHER_CTX_new() and EVP_CIPHER_CTX_free()
in libcrypto, define our own versions, with the same signatures as the
ones in OpenSSL 1.1.x's libcrypto, and have the code use
EVP_CIPHER_CTX_new() to allocate the structure and EVP_CIPHER_CTX_free()
to free it.