Thread-safe exit()

[roblabla]

We need to figure out how to make exit thread-safe. I'm getting kind-of doubtful this is possible to do entirely, but right now exit in another thread gives us a segfault. We might be able to at least give it a saner behavior.

[misson20000]

I think the best we might be able to get is calling svcCancelSynchronization on all our threads and sprinkling checks to see if we've exited all over.