Make exit() unwind properly

exit() is now internally implemented by throwing an exception,
performing a normal stack unwind and a clean shutdown. This ensures
that no persistent reso

Make exit() unwind properly

exit() is now internally implemented by throwing an exception,
performing a normal stack unwind and a clean shutdown. This ensures
that no persistent resource leaks occur.

The exception is internal, cannot be caught and does not result in
the execution of finally blocks. This may be relaxed in the future.

Closes GH-5768.

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