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_data_objects.cc

@@ -1236,7 +1236,7 @@ Event_timed::get_create_event(THD *thd, String *buf)
     append_unescaped(buf, comment.str, comment.length);
   }
   buf->append(STRING_WITH_LEN(" DO "));
-  buf->append(body.str, body.length);
+  buf->append(&body);
 
   DBUG_RETURN(0);
 }
@@ -1284,7 +1284,7 @@ Event_job_data::construct_sp_sql(THD *thd, String *sp_sql)
   */
   sp_sql->append(C_STRING_WITH_LEN("() SQL SECURITY INVOKER "));
 
-  sp_sql->append(body.str, body.length);
+  sp_sql->append(&body);
 
   DBUG_RETURN(thd->is_fatal_error);
 }