Make the parent VM always monitor its child VMs

* So at least the parent get detailed information about the termination.
* This suggests to care more about termination of created VMs.
* Adapt and improve the tests.
Fix backtrace construction with debug info

* The condition is e.debug == unit OR e.debug.stack == unit.
* Use some space for clarity instead of an extra long conditional.
Merge pull request #186 from eregon/tests

Add vm.oz to $ make test
Merge pull request #183 from eregon/fix_stacktrace_with_info

Fix backtrace construction with debug info
Merge pull request #184 from eregon/mvm

Make the parent VM always monitor its child VMs