[abstraction] Rework IRP handlers, move boot-disk probing code

[abstraction] Rework IRP handlers, move boot-disk probing code

- Changed my mind about IRP handlers; we use an IRPHandler function
  type now
- We used to probe for a MEMDISK and/or an AoE target (via aBFT)
  in the bus code; we call these functions in probe.c from
  DriverEntry() now
- driver.c handles device creation and closing now, and passes on
  any other IRPs to the device's particular Dispatch() function.
  Currently, there is only a Bus and Disk device type, where Disk
  should really be abstracted some more to split AoE from MEMDISK

All of the 'include's are looking really messy right now, but we
_should_ be on our way to being cleaner, where modules should not
need to know so much about other modules.