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.
Add vm.oz to $ make test
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
Fix comparison warnings of GCC in heap size properties

* "comparison between signed and unsigned integer expressions"
* Reported by #185
Fix a GCC uninitialized warning
Fix overflow in memory block size computations

* Do the division first. This might change the ratios by 1 * [100,199] = [100,199]
  but we are using millions of bytes so it does not matter much.
* It was unlikely to happen on 64-bit, but very likely on 32-bit!
* See #185.
Adapt GC test to have sensible heap size values
Merge pull request #187 from eregon/mvm

Fix overflow of memory management calcutations on 32-bit
merge