Make philos drop a fork when the other is taken

Make philos drop a fork when the other is taken

Instead of making a fork a mutex, it now holds its state - whether it is
taken. This makes it possible for a philo to check, whether he can grab
both forks and if they cannot, they won't hold any fork.
I thought about approach before but my mind was locked onto using the
function pthread_mutex_trylock which is not permitted by the subject.