Make string-set options behave more like bool options

Make string-set options behave more like bool options

Before this change, string-set options (like time_based) would not
complain if passed an argument, they would just always evaluate
to being set. This is very confusing if someone uses them negated,
ala:

	time_based=0

since fio would interpret that as the option being set. Now we'll
do the right thing, time_based=0 will be identical to not having
the option set. And time_based=1 or just time_based will equate
to the option being set.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>