Implement battery management sysctl. This will provide a set of sysctls

Implement battery management sysctl.  This will provide a set of sysctls
to control the charging of laptop batteries:

* hw.battery.chargemode (int)

  -1: force discharge
   0: inhibit charge
   1: auto

In auto mode charging may be controlled by:

* hw.battery.chargestop (int)

  Percentage (0-100) of last full capacity at which the battery should
  stop charging.

* hw.battery.chargestart (int)

  Percentage (0-100) of last full capacity at which the battery should
  start charging.

The idea is that with

  hw.battery.chargemode=1
  hw.battery.chargestop=80
  hw.battery.chargestart=75

the battery would be kept charged within the range between 75% and 80%.

Allowable settings and some details of the behavior may differ between
hardware implementations.

Committing this early to easy testing of further diffs that implement this
functionality in acpithinkpad(4) and aplsmc(4).

ok kn@