unix, windows: make uv_barrier_wait() return bool

Make uv_barrier_wait() return a boolean value indicating whether this
thread was the "serializer".

From `man pthread_barrier_wait`:

    Upon successful completion, the pthread_barrier_wait() function
    shall return PTHREAD_BARRIER_SERIAL_THREAD for a single (arbitrary)
    thread synchronized at the barrier and zero for each of the other
    threads.

Exposing that information from libuv is useful because it can make
cleanup significantly easier:

    if (uv_barrier_wait(&barrier) > 0)
      uv_barrier_destroy(&barrier);
Readability.