kern/semaphore: rework

The previous implementation, which uses a combination of fast and slow
paths around accessing an atomic integer, suffers from a bug triggered
when two or more posts are performed back-to-back, without a waiter
decrementing the semaphore value in between. The first post would be
the only one signalling a waiter.

In addition, having a fast path that expects the absence of waiters
probably doesn't make sense, as semaphores are expected to be used for
signalling threads. As a result, it was decided to remove the fast path
altogether, and protect the semaphore value with sleep queues.

Finally, as part of the rework, semaphores now have a user-defined maximum
value, in order to make the implementation of, e.g. wrappers for binary
semaphores, convenient.

Thanks to Simon Venken for reporting the bug.

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