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
The MR also moves
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.