MDEV-6109: EXPLAIN JSON

- Add first testcases
- Don't overquote when printing conditions
- Other small output fixes

sql/mysqld.h

@@ -611,9 +611,13 @@ enum enum_query_type
   /// Without character set introducers.
   QT_WITHOUT_INTRODUCERS= (1 << 1),
   /// view internal representation (like QT_ORDINARY except ORDER BY clause)
-  QT_VIEW_INTERNAL= (1 << 2)
+  QT_VIEW_INTERNAL= (1 << 2),
+  /// This value means focus on readability, not on ability to parse back, etc.
+  QT_EXPLAIN= (1 << 4)
 };
 
+
+
 /* query_id */
 typedef int64 query_id_t;
 extern query_id_t global_query_id;