Auto merge of #22092 - paulrouget:thread_count, r=manishearth

Count threads at shutdown, warn if not all finished and optionally block

On Desktop, after servo shutdown, the process exits, killing all the still-running threads.
On mobile, the process doesn't necessarily shutdown, and we are left with multiple threads failing to use channels, printing warning and panicking.

In the future, we'd like to do a clean shutdown, as in, shutdown only once all the threads have properly finished.

For now, I'm printing a warning if all the threads are not finished, and also add an option to only exit if all the threads have finished.

This only works on MacOS for now, in C, as I haven't found a Rust/multiplatform way to count running threads.

I recommend trying to run servo with `--clean-shutdown` option. Interesting to see how long it takes for other threads to shutdown, and see which threads start panicking.

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