Arch with systemd support

Hi,
I am a Trusted User of the ArchLinux distribution and I recently tried out ajenti.
The first issue I encountered is in the services plugin which only supports the old rc.d service system.

Arch switched to systemd a while ago, and while rc.d services are still somewhat supported, they are going to disappear with time and their usage is not encouraged.

With this commits I implemented selective support for systemd, detecting its presence by checking the pid 1 process file.

I personally would remove completely rc.d support but this solution should work for everyone.

Thanks