Improve handling of processes run from windows shell scripts

When the exec/xh-exec/xh-execbg tasks are used to execute a Windows shell script (.bat) and that script in turn executes another process, ant/xharness are unable to kill these processes. The shell skript can be terminated but the forked process generally remains running.

We should add a specialized shell-exec tasks that caters for cases where the launched process forks other processes. We can then use a 3rd party process tool (ps on Unix, pslist http://technet.microsoft.com/en-us/sysinternals/bb896682.aspx on Windows) to monitor child processes forked from the main process and make sure they are either killed when the main process terminates or are tracked and killed at a later stage (e.g. when the xharness test suite ends).