contrib/systemd: add a sysusers entry
contrib/systemd: add a sysusers entry

This adds a sysusers.d file, in order to create a system user/group
which matches the one used by the service unit.

Ref: https://www.freedesktop.org/software/systemd/man/sysusers.d.html

coreos/etcd,etcd-io/etcd,mkumatag/etcd,rnaveiras/etcd,mkumatag/etcd,mkumatag/etcd,mqliang/etcd,gyuho/etcd,etcd-io/etcd,rnaveiras/etcd,ajityagaty/etcd,coreos/etcd,rnaveiras/etcd,mqliang/etcd,ajityagaty/etcd,etcd-io/etcd,gyuho/etcd,ajityagaty/etcd,mqliang/etcd