Add `pthread_barrier_*` functions

Compare [`pthread_barrier_init`][1].

  [1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_barrier_init.html
Add pthread_barrier test
Add pthread_barrier_t documentation
Merge pull request #813 from Kijewski/add-pthread_barrier

posix: Add `pthread_barrier_*` functions