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/event_scheduler.cc

@@ -85,7 +85,7 @@ Event_worker_thread::print_warnings(THD *thd, Event_job_data *et)
   char prefix_buf[5 * STRING_BUFFER_USUAL_SIZE];
   String prefix(prefix_buf, sizeof(prefix_buf), system_charset_info);
   prefix.length(0);
-  prefix.append("Event Scheduler: [");
+  prefix.append(STRING_WITH_LEN("Event Scheduler: ["));
 
   prefix.append(et->definer.str, et->definer.length, system_charset_info);
   prefix.append("][", 2);