Add secure DSA nonce flag.

This change adds the option to calculate (EC)DSA nonces by hashing the
message and private key along with entropy to avoid leaking the private
key if the PRNG fails.

crypto/ecdsa/ecdsa.h

@@ -264,6 +264,7 @@ void ERR_load_ECDSA_strings(void);
 #define ECDSA_R_ERR_EC_LIB				 102
 #define ECDSA_R_MISSING_PARAMETERS			 103
 #define ECDSA_R_NEED_NEW_SETUP_VALUES			 106
+#define ECDSA_R_NONCE_CANNOT_BE_PRECOMPUTED		 107
 #define ECDSA_R_RANDOM_NUMBER_GENERATION_FAILED		 104
 #define ECDSA_R_SIGNATURE_MALLOC_FAILED			 105