Core: fixed a race resulting in extra sem_post()'s.

The mtx->wait counter was not decremented if we were able to obtain the lock
right after incrementing it.  This resulted in unneeded sem_post() calls,
eventually leading to EOVERFLOW errors being logged, "sem_post() failed
while wake shmtx (75: Value too large for defined data type)".

To close the race, mtx->wait is now decremented if we obtain the lock right
after incrementing it in ngx_shmtx_lock().  The result can become -1 if a
concurrent ngx_shmtx_unlock() decrements mtx->wait before the added code does.
However, that only leads to one extra iteration in the next call of
ngx_shmtx_lock().