Emulate the logind dbus API better

This patch:
- Adds the PowerOff/Reboot and respective Can APIs.
- Adds the PolkicyKit interactivity flag for the power and suspend
  API calls. Same as logind.
- Adds a note of when the API calls were introduced to the docs.
The goal is just to make it easier for consumers of the APIs to
support both ConsoleKit2 and logind.
The Stop/Restart API calls from ConsoleKit1 are still present
and function almost exactly the same (only change is they respect
inhibitors now). This patch fixes:
https://github.com/ConsoleKit2/ConsoleKit2/issues/10