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