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>

test/hmactest.c

@@ -134,7 +134,7 @@ int main(int argc, char *argv[])
     char *p;
 # endif
     int err = 0;
-    HMAC_CTX ctx, ctx2;
+    HMAC_CTX ctx = HMAC_CTX_EMPTY, ctx2 = HMAC_CTX_EMPTY;
     unsigned char buf[EVP_MAX_MD_SIZE];
     unsigned int len;