disk: gather statistics since boot on FreeBSD

disk: gather statistics since boot on FreeBSD

disk_octets and disk_ops are derives, so we don't want rates
but absolute values.

I tested this with fio with a constant IO rate and confirmed
that the values are correct for disk_octets and disk_ops.

disk_time is a different matter and I need some help with that.
devstat_compute_statistics returns a long double and it didn't
seem to increase much on my system, but that might be because I
tested this with a fast SSD.