Added return(0) to the Mac/IOKit variant of battery_read()
prevent going through systemd/upstart code, except on Linux

Fixes #809 (build issue on MacOSX)

NB: in case one day upstart is used on non-Linux platforms, this could
be relaxed to only skip systemd.
Merge pull request #810 from njh/mac_battery_read_return

Added return (0) to the Mac/IOKit variant of battery_read()
Merge pull request #811 from mfournier/systemd-upstart-build-issue

prevent going through systemd/upstart code, except on Linux