added systemd unit for easier server-deployment

this might might make things easier for people who wish to deploy their own ioquake3-servers on a systemd-supporting host.
The execstart line is correct but this seems to give misleading information about the dedicated cvar.
dedicated 0 is not allowed in the ded bin (run as a listen server with client)
dedicated 1 - dedicated but lan only
dedicated 2 - dedicated internet