Fixed problem with (load "cpu.lisp").

On systems with new sysfs interface of thermal zone
I received an error in (defvar *acpi-thermal-zone* ...), so
I made function can recognize what interface is used(/proc or /sys)
and fetch info from the right place without annoying errors.
Also I found that bar function from mode-line isn't exported, therefore
fmt-cpu-usage-bar throws error.