- 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_signal.cc

@@ -89,11 +89,11 @@ void Set_signal_information::clear()
   memset(m_item, 0, sizeof(m_item));
 }
 
-void Sql_cmd_common_signal::assign_defaults(
-                                    Sql_condition *cond,
-                                    bool set_level_code,
-                                    Sql_condition::enum_warning_level level,
-                                    int sqlcode)
+void
+Sql_cmd_common_signal::assign_defaults(Sql_condition *cond,
+                                       bool set_level_code,
+                                       Sql_condition::enum_warning_level level,
+                                       int sqlcode)
 {
   if (set_level_code)
   {