Darwin: remove global variable used in aio_return

Darwin: remove global variable used in aio_return

This was using a global variable to signify a read success
between pre- and post- aio_return. I don't think that this
was safe. There could be multiple calls to aio_return on
different threads. The global aiocpb_table is safe enough
as the pointers to control block are unique. However if two
threads manipulate was_a_successful_aio_read there's no way
of telling them apart. Rather than trying to make
was_a_successful_aio_read thread safe I did the same as on
FreeBSD and removed post- aio_return and now do everything
in pre- aio_return. No global variable, no thread hazard.