* changed return value in case of failure in ThreadManager.SemaphoreInit
    from nil to pointer(-1), because nil/0 is a valid return value for
    sem_open() on some platforms

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26079 3ad0048d-3df7-0310-abae-a5850022a9f2