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_load.cc

@@ -837,7 +837,7 @@ static bool write_execute_load_query_log_event(THD *thd, sql_exchange* ex,
       if (n++)
         query_str.append(STRING_WITH_LEN(", "));
       append_identifier(thd, &query_str, item->name.str, item->name.length);
-      query_str.append(val->name.str, val->name.length);
+      query_str.append(&val->name);
     }
   }