options: ensure that we check all candidates on is_set check

We can have multiple options that touch the same variable. One
example is cpus_allowed or cpumask, both will set the same
backend variable. But if we set cpumask= and later check if
cpus_allowed is set, then we want that to report true.

This breaks certain options since b2a9e6496494.

Reported-by: Vincent Fu <Vincent.Fu@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Fio 2.2.3

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