posix/pthreads: fix pthread_barrier_wait() behavior to match Posix

Exactly one caller of pthread_barrier_wait() should receive a return
value of PTHREAD_BARRIER_SERIAL_WAIT; all others should receive zero
(or an error code). Added a test to match.

Fixes: #9953

Signed-off-by: Charles E. Youse <charles.youse@intel.com>