Add systemd timer example for nncp-clean

I've recently written an Ansible role for deploying ("server-mode") NNCP and wanted to contribute the systemd units upstream.

One major change I make from your setup is that I moved the spool into /var/lib/nncp and use /var/log/nncp/$PROGRAM.log for individual parts of NNCP (e.g. /var/log/nncp/daemon.log). Also the services are heavily sandboxed - see for instance nncp-daemon. But this change is intended for the setup from this repository 🙂 If there's interest in integrating these in the units here I can make PRs for it