Fix truncation of comparison value introduced in 848acd67.

Casting the difference of the two off_t values to size_t may truncate
the result. Doing so before the comparison will therefore break it.
Instead cast the size to off_t to get around the signed versus unsigned
integer expression comparison and then cast the result of the comparison
to size_t again. Should fix #9714.
acpi_cpuidle.cpp: Some style cleanup

Just what I spotted at the first glance. There's more general cleanup
to be done, e.g. regarding the public API.
acpi_cpuidle: Fix driver initialization

* For some reason the code in acpi_processor_init() assumed that the
  CPU index returned by ACPI was 1-based. On my machines it is 0-based
  and the syslogs on Trac from the last months don't show any
  exception. The code even contained a hack to force satisifying that
  assumption for a single-CPU machine.
* Instead of calling intel_cpuidle_init()/acpi_cpuidle_init() when
  initializing the driver for the device node for CPU 0 -- thus
  depending on the order the device manager iterates through the CPU
  device nodes -- rather check whether all CPU nodes have indeed been
  encountered.
* acpi_cpuidle_init_driver(): Don't leak the cookie in error case.
* acpi_cpuidle_uninit_driver(): Clear the sAcpiProcessor element for
  the uninitialized CPU node. We'd also have to unregister the idle
  callback when acpi_cpuidle_uninit_driver() is called for the first
  CPU device node, but there isn't any API for it ATM.

Fixes a crash on machines with multiple CPUs where acpi_cpuidle_setup()
wouldn't already fail early (apparently only mine).
Remove x86_cpuidle driver from image

Apparently it isn't ready yet. I haven't found a single syslog on Trac
that shows a successful initialization of the driver, and on my
machine, where it does that at least, the system freezes during boot.