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/statem/statem_srvr.c

@@ -3151,7 +3151,7 @@ int tls_construct_new_session_ticket(SSL *s)
 {
     unsigned char *senc = NULL;
     EVP_CIPHER_CTX ctx;
-    HMAC_CTX hctx;
+    HMAC_CTX hctx = HMAC_CTX_EMPTY;
     unsigned char *p, *macstart;
     const unsigned char *const_p;
     int len, slen_full, slen;