by Andy Doran. Also document the get/set pshared thread calls as not
implemented, and add a skeleton implementation that is disabled.
XXX: document _sched_protect(2).
- pthread_barrier_{init,destroy,wait} into pthread_barrier(3).
- pthread_cond_* into pthread_cond(3).
- pthread_mutex_* into pthread_mutex(3).
- pthread_rwlock_* into pthread_rwlock(3).
- pthread_spin_* into pthread_spin(3).
Cross-link amongst groups.
OK jruoho@