handling of savepoints in stored routines.

Fixed ha_rollback_to_savepoint()/ha_savepoint()/ha_release_savepoint()
functions to properly handle savepoints inside of stored functions and
triggers.
Also now when we invoke stored function or trigger we create new savepoint
level. We destroy it at the end of function/trigger execution and return back
to old savepoint level.


mysql-test/r/sp_trans.result:
  Added test for bug #13825 "Triggers: crash if release savepoint" and for
  general handling of savepoints in stored routines.
mysql-test/t/sp_trans.test:
  Added test for bug #13825 "Triggers: crash if release savepoint" and for
  general handling of savepoints in stored routines.
sql/ha_innodb.cc:
  innobase_savepoint():
    Replaced check which always failed due to similar check in caller
    with assertion.
sql/handler.cc:
  ha_rollback_to_savepoint()/ha_savepoint()/ha_release_savepoint():
    Changed functions to properly support handling of savepoints
    inside of stored functions and triggers.
sql/sql_class.cc:
  THD::reset_sub_statement_state()/restore_sub_statement_state():
    When we invoke stored function or trigger we should create new savepoint
    level. We should destroy it at the end of function/trigger execution and
    return back to old savepoint level. To support this behavior we should
    save and reset list of current savepoints on entering function and restore
    old list when we leave it.
sql/sql_class.h:
  Sub_statement_state:
    When we invoke stored function or trigger we should create new savepoint
    level. We should destroy it at the end of function/trigger execution and
    return back to old savepoint level. To support this behavior added "savepoint"
    member which is used to save/restore list of current savepoints on
    entering/leaving function.
sql/sql_parse.cc:
  mysql_execute_command():
    Changed processing of SQLCOM_SAVEPOINT so now it is not ignored when
    we are in autocommit mode and savepoint is set inside of stored
    function or trigger.