Pack of CAM improvements

Hi.

I've made a patch, that should solve set of problems of CAM ATA and CAM
generally. I would like to ask for testing and feedback.

What patch does:
- It unifies bus reset/probe sequence. Whenever bus attached at boot or
later, CAM will automatically reset and scan it. It allows to remove
duplicate code from many drivers.
- Any bus, attached before CAM completed it's boot-time initialization,
will equally join to the process, delaying boot if needed.
- New kern.cam.boot_delay loader tunable should help controllers that
are still unable to register their buses in time (such as slow USB/
PCCard/ CardBus devices).
- To allow synchronization between different CAM levels, concept of
requests priorities was extended. Priorities now split between several
"run levels". Device can be freezed at specified level, allowing higher
priority requests to pass. For example, no payload requests allowed,
until PMP driver enable port. ATA XPT negotiate transfer parameters,
periph driver configure caching and so on.
- Frozen requests are no more counted by request allocation scheduler.
It fixes deadlocks, when frozen low priority payload requests occupying
slots, required by higher levels to manage theit execution.
- Two last changes were holding proper ATA reinitialization and error
recovery implementation. Now it is done: SATA controllers and Port
Multipliers now implement automatic hot-plug and should correctly
recover from timeouts and bus resets.

Patch can be found here:
http://people.freebsd.org/~mav/cam-ata.20100119.patch

Feedback as always welcome.

-- 
Alexander Motin