systemd: use kresd.target for service start

Based on discussion in upstream systemd ticket #9080, it seems the conceptually right way to start multiple instances of the service is to use a target, instead of using globbing in systemctl start.

The MR also moves systemd/ to distro/common/systemd to clean up the directory tree.

@dkg is it all right to enable the empty kresd.target by default during package installation in debian? It seems to be the most user-friendly way to handle it.