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);

Signed-off-by: Fedor Indutny <fedor@indutny.com>
include: document barrier functions

Add some basic doc comments to the uv_barrier_*() functions and document
the "serializer" return value from the previous commit.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
test: make barrier test more rigorous

A correct barriers implementation blocks in uv_barrier_destroy() until
the last thread returns from uv_barrier_wait() so make the test more
rigorous by destroying the barrier straight away instead of first
joining the worker thread.

Signed-off-by: Fedor Indutny <fedor@indutny.com>