Adapt HMAC to the EVP_MD_CTX changes

This change required some special treatment, as HMAC is intertwined
with EVP_MD.  For now, all local HMAC_CTX variables MUST be
initialised with HMAC_CTX_EMPTY, or whatever happens to be on the
stack will be mistaken for actual pointers to EVP_MD_CTX.  This will
change as soon as HMAC_CTX becomes opaque.

Also, since HMAC_CTX_init() can fail now, its return type changes from
void to int, and it will return 0 on failure, 1 on success.

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

ssl/t1_lib.c

@@ -3041,7 +3041,7 @@ static int tls_decrypt_ticket(SSL *s, const unsigned char *etick,
     const unsigned char *p;
     int slen, mlen, renew_ticket = 0;
     unsigned char tick_hmac[EVP_MAX_MD_SIZE];
-    HMAC_CTX hctx;
+    HMAC_CTX hctx = HMAC_CTX_EMPTY;
     EVP_CIPHER_CTX ctx;
     SSL_CTX *tctx = s->initial_ctx;
     /* Need at least keyname + iv + some encrypted data */