MWL#182: Explain running statements: address review feedback

- switch SHOW EXPLAIN to using an INFORMATION_SCHEMA table.

sql/sql_lex.h

@@ -2357,7 +2357,8 @@ struct LEX: public Query_tables_list
   char *backup_dir;				/* For RESTORE/BACKUP */
   char* to_log;                                 /* For PURGE MASTER LOGS TO */
   char* x509_subject,*x509_issuer,*ssl_cipher;
-  String *wild;
+  String *wild; /* Wildcard in SHOW {something} LIKE 'wild'*/ 
+  Item *show_explain_for_thread; /* id in SHOW EXPLAIN FOR id */
   sql_exchange *exchange;
   select_result *result;
   Item *default_value, *on_update_value;