Change default script stop method to co-op instead of abort.

co-op should be more stable as it doesn't abort threads, which can trigger virtual machine instability
This change will be invisible to users as script DLLs are recompiled automatically where necessary, though the change won't take affect until the next simulator restart.
This change has no effect on existing script state.
If you want to continue using abort, set ScriptStopStrategy = abort in the [XEngine] section of OpenSim.ini