- 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_array.h

@@ -36,16 +36,16 @@ template <typename Element_type> class Bounds_checked_array
 public:
   Bounds_checked_array() : m_array(NULL), m_size(0) {}
 
-  Bounds_checked_array(Element_type *el, size_t size)
-    : m_array(el), m_size(size)
+  Bounds_checked_array(Element_type *el, size_t size_arg)
+    : m_array(el), m_size(size_arg)
   {}
 
   void reset() { m_array= NULL; m_size= 0; }
  
-  void reset(Element_type *array, size_t size)
+  void reset(Element_type *array_arg, size_t size_arg)
   {
-    m_array= array;
-    m_size= size;
+    m_array= array_arg;
+    m_size= size_arg;
   }
 
   /**