An MMC/SD/SDIO stack using CAM

Implement the MMC/SD/SDIO protocol within a CAM framework. CAM's
flexible queueing will make it easier to write non-storage drivers
than the legacy stack. SDIO drivers from both the kernel and as
userland daemons are possible, though much of that functionality will
come later.

Some of the CAM integration isn't complete (there are sleeps in the
device probe state machine, for example), but those minor issues can
be improved in-tree more easily than out of tree and shouldn't gate
progress on other fronts. Appologies to reviews if specific items
have been overlooked.

Submitted by: Ilya Bakulin
Reviewed by: emaste, imp, mav, adrian, ian
Differential Review: https://reviews.freebsd.org/D4761

merge with first commit, various compile hacks.