Implement memset_s() and use it instead of zero_bytes().

A new constant, SUDO_CONV_REPL_MAX, is defined by the plugin
API as the max conversation reply length.  This constant can be
used as a max value for memset_s() when clearing passwords
filled in by the conversation function.

plugins/sudoers/defaults.c

@@ -356,7 +356,7 @@ init_defaults(void)
 		    list_op(NULL, 0, def, freeall);
 		    break;
 	    }
-	    zero_bytes(&def->sd_un, sizeof(def->sd_un));
+	    memset(&def->sd_un, 0, sizeof(def->sd_un));
 	}
     }