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.h

@@ -339,6 +339,7 @@ enum def_tuple {
     sudo,
     json,
     json_compact,
+    json_pretty,
     dso,
     trace
 };