systemd power control support

My other Arch box has Systemd installed and doesn't have PMUtils, nor I need them, as Systemd handles power control well enough.

With this patch I add Systemd power management support, as well as pluggable power management interface. If you don't like the idea of incorporating Systemd power management into Ajenti core, it can be moved out of it into a third-party plugin, but please consider leaving `PowerController` interface in place in the case, so I (or anybody else) can extend on it.

Thank you.