Add functions to read the ACPI status from the /sys tree in Linux.

In addition to adding the ACPI functions I have wired them into my
status output to have a plainly visible battery meter that is refreshed
when the time is.  The formatting is trivial to change, and fairly
straightforward.

The readfile() and getbattery() functions were taken from the "Helper
Functions" section of the dwmstatus website.  getonac() was written by
me so that I could have an indicator of when I was on battery and when
I was on A/C.  It is based on an understanding of what getbattery()
does, and reads from the same /sys tree.

I am not sure that this code will work for other Unix-like operating
systems, so please let me know if they do if you use another Unix-like
system.  Otherwise I would be open to writing something more substatial
that works on other systems.