options: add support for checking if an option has been set

options: add support for checking if an option has been set

We often just do a non-zero check, but an option can be set
and be set to zero, so we can't currently detect that. For
complicated options, we add a second variable to mark if it
has been set or not. This allows us to directly check if a
given option was set by the user or not.

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