Expose a sysctl interface hw.acpi.thinkpad<M>.bat[<N>].<behavior> to control
some aspects of battery charging behavior on supported systems:

    charge_start
threshold below which to start charging (in %, 0-99)

    charge_stop
threshold above which to stop charging (in %, 1-100)

    force_discharge
discharge while on AC power, e.g., for calibration

    charge_inhibit
inhibit charging while on AC power

From Malte Dehling

(christos)