Replace redundant TD_F_NOIO flag with td->io_ops_init

The only reason TD_F_NOIO exists in addition to FIO_NOIO is because
fio_running_or_pending_io_threads() needs to know whether td has
successfully initialized ioengine, whereas FIO_NOIO is statically
set regardless of ioengines's ->init() result.

This commit adds a new td field ->io_ops_init to inidicate ->init()
result, so that td needs no extra bit field for each ioengine like
TD_F_NOIO. It was rather odd that td was unable to tell ->init()
result afterward when ->init() failure (returning non zero) doesn't
mean aborting fio itself. This commit also changes TD_F_NOIO to
TD_F_RESERVED as it's no longer used.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>