cpupower: Implement disabling of cstate interface

Latest kernel allows to disable C-states via:
/sys/devices/system/cpu/cpuX/cpuidle/stateY/disable

This patch provides lower level sysfs access functions to make use of
this interface.  A later patch will implement the higher level stuff.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>