- Renaming variables so that they don't shadow others (After this patch one can compile with -Wshadow and get much fewer warnings)

- Changed ER(ER_...) to ER_THD(thd, ER_...) when thd was known or if there was many calls to current_thd in the same function.
- Changed ER(ER_..) to ER_THD_OR_DEFAULT(current_thd, ER...) in some places where current_thd is not necessary defined.
- Removing calls to current_thd when we have access to thd

Part of this is optimization (not calling current_thd when not needed),
but part is bug fixing for error condition when current_thd is not defined
(For example on startup and end of mysqld)

Notable renames done as otherwise a lot of functions would have to be changed:
- In JOIN structure renamed:
   examined_rows -> join_examined_rows
   record_count -> join_record_count
- In Field, renamed new_field() to make_new_field()

Other things:
- Added DBUG_ASSERT(thd == tmp_thd) in Item_singlerow_subselect() just to be safe.
- Removed old 'tab' prefix in JOIN_TAB::save_explain_data() and use members directly
- Added 'thd' as argument to a few functions to avoid calling current_thd.

sql/sql_delete.cc

@@ -312,7 +312,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
   if (safe_update && const_cond)
   {
     my_message(ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE,
-               ER(ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE), MYF(0));
+               ER_THD(thd, ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE), MYF(0));
     DBUG_RETURN(TRUE);
   }
 
@@ -437,7 +437,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
       delete select;
       free_underlaid_joins(thd, select_lex);
       my_message(ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE,
-                 ER(ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE), MYF(0));
+                 ER_THD(thd, ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE), MYF(0));
       DBUG_RETURN(TRUE);
     }
   }