enable stacked monitoring scripts

The Job Scheduler should enable multiple scripts in different scripting languages that are processes as monitors.

The configuration should look like this:

<monitor>
<script language="javascript">// script 1 ....</script>
<script language="perlscript"># script 2 ....</script>
<script language="javascript">// script 3 ....</script>
</monitor>

For the methods spooler_task_before() and spooler_process_before() the following rules apply:

• the sequence of processing is given by the sequence in the above configuration.
• if any of the monitor scripts returns false then processing of other monitor scripts is prevented.

For the methods spooler_task_after() and spooler_process_after() the following rules apply:

• the sequence of processing is vice versa to the sequence in the above configuration.
• all monitor scripts are processed, independently of the respective return value
• the return value of a monitor script is the argument for the subsequent monitor script