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);
Signed-off-by: Fedor Indutny <fedor@indutny.com>
91985d48f6