Forking a multithreaded process is dangerous but allowed, so long as the child
only executes async-signal-safe functions (e.g. exec). Add a test to ensure
that we don't break this behavior.
When witness is enabled, lock rank order needs to be preserved during
prefork, not only for each arena, but also across arenas. This change
breaks arena_prefork into further stages to ensure valid rank order
across arenas. Also changed test/unit/fork to use a manual arena to
catch this case.
