Get the ECDH cofactor mode from a key context.
Declared in <openssl/ec.h>
<openssl/ec.h>
int EVP_PKEY_CTX_get_ecdh_cofactor_mode(EVP_PKEY_CTX* ctx);
Cofactor mode (0 = disable, 1 = enable, -1 = use key default), or a negative error code.