Add a battery plugin that uses the modern Linux locations for battery in...

...formation

<https://github.com/collectd/collectd/issues/424>

The existing plugin uses `/proc/acpi/battery`.  Linux no longer keeps
battery information here.

This patch adds an alternative battery plugin which uses the modern
location

    /sys/class/power_supply/BATx/

It finds the first directory that exists for BATx.