mutex: add __fio_mutex_remove()

We have to remember to free the condvar, so add a __fio_mutex_remove()
for cases that init a mutex inside an smalloc'ed region with
__fio_mutex_init().

Signed-off-by: Jens Axboe <axboe@fb.com>
options: add debug code for failure to lookup option names

Signed-off-by: Jens Axboe <axboe@fb.com>