do sem_unlink before sem_open

[maho]

This is fix for #95. I hope proper one.

Inspiration: https://stackoverflow.com/questions/8063613/c-macs-os-x-semaphore-h-trouble-with-sem-open-and-sem-wait

[koekeishiya]

Fixed on master. I opted to unlink the semaphore in the running instance that creates it, as there is no need to have this semaphore be visible to other processes.