- we should probably extend pthread_mutexattr_t with a field to create a

  single linked list of all instances.  This requires changing the

  pthread_mutexattr_* functions.

- a new attribute for mutexes: number of times we spin before calling

sys_futex

- for adaptive mutexes: when releasing, determine whether somebody spins.

If yes, for a short time release lock.  If someone else locks no wakeup

syscall needed.

- test with threaded process terminating and semadj (?) being applied

  only after all threads are gone

- semaphore changes:

  - sem_post should only wake one thread and only when the state of

    the semaphore changed from 0 to 1

    this also requires that sem_wait and sem_timedwait don't drop the

    post if they get canceled.

  - possibly add counter field.  This requires reviving the

    differences between old and new semaphose funtions.  The old ones

    stay as they are now.  The new once can use an additional field

    wich is the counter for the number of waiters