Add support for getting real-time acpi battery events from devd. This
won't work yet for non-root users unless they are using my devd patch
at http://www.marcuscom.com/downloads/devd.diff to give open permissions
to the devd.pipe socket.
Without this real-time update support, it can take up to 30 seconds for the
battstat applet to update on an ACPI event. Because this support is still
experimental, I am not bumping PORTREVISION.