targetcli-fb: import systemd file from fedora

taken from
http://pkgs.fedoraproject.org/cgit/targetcli.git/tree/targetcli.service
@1fdb82ada22d627360f48b7c71db452c7d81a9e7
targetcli-fb: move to target.service, include

target.service should now work as a systemd service.
rtslib-fb >= fb28 is needed.

The old sysvinit rc-script is still provided for now.