- 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_parse.h

@@ -70,9 +70,9 @@ LEX_USER *create_default_definer(THD *thd, bool role);
 LEX_USER *create_definer(THD *thd, LEX_STRING *user_name, LEX_STRING *host_name);
 LEX_USER *get_current_user(THD *thd, LEX_USER *user, bool lock=true);
 bool sp_process_definer(THD *thd);
-bool check_string_byte_length(LEX_STRING *str, const char *err_msg,
+bool check_string_byte_length(LEX_STRING *str, uint err_msg,
                               uint max_byte_length);
-bool check_string_char_length(LEX_STRING *str, const char *err_msg,
+bool check_string_char_length(LEX_STRING *str, uint err_msg,
                               uint max_char_length, CHARSET_INFO *cs,
                               bool no_error);
 CHARSET_INFO* merge_charset_and_collation(CHARSET_INFO *cs, CHARSET_INFO *cl);