Add "json_pretty" log format, currently the same as "json".

In a future version, "json" will be an alias for "json_compact"
instead.  GitHub issue #357.

plugins/sudoers/def_data.c

@@ -42,6 +42,7 @@ static struct def_values def_data_log_format[] = {
     { "sudo", sudo },
     { "json", json },
     { "json_compact", json_compact },
+    { "json_pretty", json_pretty },
     { NULL, 0 },
 };