get_value for semaphore

[cheme]

I happen to need a sem_getvalue like fn for semaphore, this should do.
With other approach 0 should be return for negative value, which is not really what I needed (see linked code).

[nrc]

r? @alexcrichton

[alexcrichton]

I'm somewhat wary about providing this method on semaphores as it's inherently a racy piece of information to inspect about a semaphore. There are perhaps some situations where when you see a particular number you know a particular fact, but in general once you see the value of a semaphore you don't actually know anything for certain.

For example the code snipped in the documentation is racy in that it may not actually unlock the semaphore a correct number of times.

I think for now we may wish to be more conservative about the API.

[cheme]

Yes, you're right, I think I felt a bit lazy by using a semaphore this way. Guess I'll stick to a condvar + int mutex in my code.