Never start systemd services in background

Never start systemd services in background

Only invoke (specific) services in background when not running
systemd, otherwise services just might not get started. There
seems to be a race-condition in the init script of gpm which
doesn't properly run under systemd when invoked via "systemctl
start gpm" in background.  This should be solved once there's a
proper gpm systemd unit file.

Thanks: Leo Bergolth for the bugreport

Closes grml/grml#76