Make battery emptying rate available as sysctl variable.

Curiously, the in-kernel routines always use the design voltage to
convert from mA to mW, but acpiconf in userland uses the current
voltage. As a result, this can report a different mW rate than
acpiconf.

Submitted by: Manuel Stühn <freebsdnewbie@freenet.de>
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D17077