linked-list: Change return value of find_first() and signature of its callback

This avoids the unportable five pointer hack.

src/libstrongswan/credentials/credential_manager.c

@@ -812,11 +812,12 @@ static bool verify_trust_chain(private_credential_manager_t *this,
 	return trusted;
 }
 
-/**
- * List find match function for certificates
- */
-static bool cert_equals(certificate_t *a, certificate_t *b)
+CALLBACK(cert_equals, bool,
+	certificate_t *a, va_list args)
 {
+	certificate_t *b;
+
+	VA_ARGS_VGET(args, b);
 	return a->equals(a, b);
 }
 
@@ -896,8 +897,7 @@ METHOD(enumerator_t, trusted_enumerate, bool,
 			continue;
 		}
 
-		if (this->failed->find_first(this->failed, (void*)cert_equals,
-									 NULL, current) == SUCCESS)
+		if (this->failed->find_first(this->failed, cert_equals, NULL, current))
 		{	/* check each candidate only once */
 			continue;
 		}