cam-iosched: Publish parameters of the latency buckets

Add sysctls to publish the latency bucket size, number, and stride. Move
to putting all the iosched stuff under kern.cam.iosched as well and move
kern.cam.do_dynamic_iosched to kern.cam.iosched.dynamic. In addition, move
kern.cam.io_sched_alpha_bits to kern.cam.iosched.alpha_bits. Publish
kern.cam.iosched.bucket_base (the smallest bucket time), .bucket_ratio
(the geometric progression factor * 100), and .buckets (the total number
of buckets).

Move to publishing 20 buckets, starting at 20us. This allows us to get
better resolution on the short end and detect preformance degredation of
the NVMe drives I've tested on, even with the uncertainty of bucketing.

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