Explicitly return thread interrupt state in zstream_run_func

[ianks]

Previously, there were race conditions checking the interrupt flag when checking whether to retry zstream_run_func. Instead of relying on the volatile args->interrupt state, we now explicitly signal the function returned due to an interrupt so it can be retried safely.

fixes #57

[ioquatix]

@ianks just curious why you closed this?

[ioquatix]

Ian, I merged the other PR, sorry, did not mean to cause you extra work. Let's try it and see if we need to iterate again.