Previously process::exit() was going through the threads in order
calling thread::exit() - which blocks and never wakes if called on the
current thread. Since the current thread likely belongs to the process
which is exiting, and the current thread wasn't guaranteed to be last in
the list, this could leave threads not cleaned up.
Worse, no matter what, this caused the m_threads_lock to always be held
forever on exit, keeping the scheduler from ever finishing a call to
process::thread_exited() on its threads.
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