Make EVP_CIPHER_CTX opaque and renew the creator / destructor functions

Following the method used for EVP_MD_CTX and HMAC_CTX,
EVP_CIPHER_CTX_init and EVP_CIPHER_CTX_cleanup are joined together
into one function, EVP_CIPHER_CTX_reset, with EVP_CIPHER_CTX_init kept
as an alias.
EVP_CIPHER_CTX_cleanup fills no purpose of its own any more and is
therefore removed.

Reviewed-by: Rich Salz <rsalz@openssl.org>