src/common.[ch]: Implement strtogauge().
battery plugin: Import code to read metrics from sysfs.

This started as a simple import of Andy Parkins' sysfsbattery plugin into
the battery plugin. Since the battery plugin is ancient and hasn't been
touched in a while, this quickly escalated to a much bigger refactoring.
Sorry!

On the other hand, this fixes a couple of bugs. For example, all metrics
were always dispatched with plugin_instance "0". This is correct for the
majority of laptops, of course, but in theory this could be wrong.

Also ACPI charging / discharging rate is reported as "current", when
modern batteries actually report "power". The sysfs code does this
correctly, ACPI still needs to be patched.

Fixes: #725