Lots of small cleanups

- Simplified use_trans_cache() to return at once if is_transactional is set
- Indentation and spelling errors fixed
- Don't call signal_update() if update_binlog_end_pos() is called as the
  function already calls signal_update()
- Removed not used function wait_for_update_bin_log(), which would cause
  errors if ever used.
- Simplified handler::clone() by always allocating 'ref' in ha_open(). To do
  this I added an optional MEM_ROOT argument to ha_open() to be used when
  allocating 'ref'
- Changed arguments to get_system_var() from LEX_CSTRING to LEX_CSTRING*
- Added THD as argument to create_select_for_variable(). Changed also char*
  argument to LEX_CSTRING to avoid strlen() call.
- Change calls to append() to use LEX_CSTRING

sql/sql_parse.h

@@ -93,7 +93,7 @@ void mysql_parse(THD *thd, char *rawbuf, uint length,
                  Parser_state *parser_state, bool is_com_multi,
                  bool is_next_command);
 bool mysql_new_select(LEX *lex, bool move_down, SELECT_LEX *sel);
-void create_select_for_variable(const char *var_name);
+void create_select_for_variable(THD *thd, LEX_CSTRING *var_name);
 void create_table_set_open_action_and_adjust_tables(LEX *lex);
 void mysql_init_multi_delete(LEX *lex);
 bool multi_delete_set_locks_and_link_aux_tables(LEX *lex);