Allow rescheduling in a mipscall, not a callback.
Simplify argument passing for mipscalls.
Return of mipscalls should be handled by actions.
Loading a savestate crashed with active mipscalls.
Oops, still need to switch back from interrupts.

There's the real problem: don't leave the thread "running".
When creating threads, etc. restore thread ready.

The old thread was sticking in the RUNNING state.
Merge pull request #574 from unknownbrackets/callbacks

Don't consider mipscalls proper callbacks