Fix 5771/library: in pthread_exit(), delay the call to _sem_post() that

Fix 5771/library: in pthread_exit(), delay the call to _sem_post() that
unblocks the pthread_join() for this thread until the exiting thread is
completely done with its thread structure, as the joining thread will
free it once unblocked.  Also, don't bother to call _sem_post() if the
thread is detached.

ok kurt@