Expose bif_capacity as the 'design capacity' through an acpibat sensor,

Expose bif_capacity as the 'design capacity' through an acpibat sensor,
giving you the original capacity of the battery:

hw.sensors.acpibat0.watthour0=64.07 Wh (last full capacity)
hw.sensors.acpibat0.watthour4=84.24 Wh (design capacity)

hw.sensors.acpibat0.amphour0=2.03 Ah (last full capacity)
hw.sensors.acpibat0.amphour4=4.40 Ah (design capacity)

Diff from Fabian Raetz on tech@, thanks!

ok mlarkin@ zhuk@ armani@ kettenis@