targetcli-fb: systemd support

The systemd service file is taken from Fedora.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>