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>

include/openssl/hmac.h

@@ -77,9 +77,10 @@ typedef struct hmac_ctx_st {
     unsigned char key[HMAC_MAX_MD_CBLOCK];
 } HMAC_CTX;
 
+# define HMAC_CTX_EMPTY      { NULL, NULL, NULL, NULL, 0, "" }
 # define HMAC_size(e)    (EVP_MD_size((e)->md))
 
-void HMAC_CTX_init(HMAC_CTX *ctx);
+int HMAC_CTX_init(HMAC_CTX *ctx);
 void HMAC_CTX_cleanup(HMAC_CTX *ctx);
 
 #ifdef OPENSSL_USE_DEPRECATED