fix futex usage and shrinking counter

* sleeping workers have decremented the semaphore count before sleeping.
  But if they are notified specifically the semaphore counter is
  decremented excessively
  This results in unnecessary suspension/notifications because the
  counter is out of sync with the actual waiter count.
* waitv expects that the futex size is specified in the futex flags
* wake sleepers using FUTEX_PRIVATE_FLAG
* futex_waitv returns the index of the woken futex -> wake on ret > -1
* add debug output and asserts