Add Service Introspection

Short version: this allows us to return high data that describes the services that are running and the services that are enabled on a machine.

Currently only systems running pacman and systemd are supported. However, this lays out an example of how to do other package managers and init systems. At this point only systemd matters going forward (as far as Linux init systems are concerned), but I expect people will still have a need to introspect legacy upstart and sysv init systems. I will likely add rpm support in the very near future, but somebody else will need to do support for dpkg and any others. Likewise, I'm probably not going to write sysv or upstart myself.