Mac sema

I made an attempt at implementing the MAC unnamed semaphors using dispatch semaphores as suggested by the stack overflow article mentioned in issue #125 (closed) . Unfortunately, my virtual machine doesn't seem to be able to create sound (no sound device is present), so I can't tell if it works at all. Also, on the virtual machine, frotz seems to hang on exit with this change. My guess is that it is a problem that a thread is waiting on a semaphore, but it could be that the issue is with the missing sound device. I thought I would put it out there so others can see whatever silly mistake I am making.

Please do not merge this as it is unless we can prove that the hang is an issue with my virtual machine.