Complete semaphore.h implementation

Spec link: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/semaphore.h.html

Constants:

• SEM_FAILED
• O_CREAT (from fcntl.h)
• O_EXCL (from fcntl.h)

Functions:

• sem_clockwait
• sem_close
• sem_destroy
• sem_getvalue
• sem_init
• sem_open
• sem_post
• sem_timedwait
• sem_trywait
• sem_unlink
• sem_wait