Fix an assertion noted by Jussi Kivilinna.

This was a consequence of the recent robust mutex changes.  If robust mutexes are selected, then each mutex that a thread takes is retained in a list in threads TCB.  If the thread exits and that list is not empty, then we know that the thread exitted while holding mutexes.  And, in that case, each will be marked as inconsistent and the any waiter for the thread is awakened.

For the case of pthread_mutex_trywait(), the mutex was not being added to the list!  while not usually a fatal error, this was caught by an assertion when pthread_mutex_unlock() was called:  It tried to remove the mutex from the TCB list and it was not there when, of course, it shoule be.

The fix was to add pthread_mutex_trytake() which does sem_trywait() and if successful, does correctly add the mutext to the TCB list.  This should eliminated the assertion.