MDEV-3798: EXPLAIN UPDATE/DELETE

- Fix a problem with EXPLAIN multi_table UPDATE: 
  = Do use multi_update object, because multi_update::prepare() does
    various setup, e.g. it disables index-only for the tables to be updated.
  = Protect multi_update::prepare() from being invoked multiple times. 
    If the query has subqueries, they may try to invoke it, for some reason.

sql/sql_class.h

@@ -4181,7 +4181,9 @@ class multi_update :public select_result_interceptor
      so that afterward send_error() needs to find out that.
   */
   bool error_handled;
-
+  
+  /* Need this to protect against multiple prepare() calls */
+  bool prepared;
 public:
   multi_update(TABLE_LIST *ut, List<TABLE_LIST> *leaves_list,
 	       List<Item> *fields, List<Item> *values,