options: change --volstep to 2

With default settings, this allows you to hit the 100% mark (with
default --softvol-max in the middle) even if you've reached min or max
volume before. This is because 50 is not divisible by 3 (old default)
but by 2 (new default).

Not really sure why there still can be issues with higher --softvol-max
and --volstep=1, but this is where I stop caring.