correct end_log_pos for Xid_log_event


mysql-test/r/binlog.result:
  don't depend on the previous tests
mysql-test/r/rpl_rotate_logs.result:
  correct end_log_pos for Xid_log_event
mysql-test/t/binlog.test:
  don't depend on the previous tests
sql/handler.h:
  comment
sql/log_event.cc:
  advance position for Xid
sql/log_event.h:
  comment
sql/sql_class.h:
  correct end_log_pos for Xid_log_event
sql/sql_parse.cc:
  make sure commit handler knows whether it's autocommit or not