Slim poller improvements

[jacobbaungard]

A couple of general improvements related to slim pollers, summary below, see individual commits for details

• Add containerhealth mon command, to avoid shipping it directly in container images
• Adjust restart.sh so it works on systems without systemd, avoiding the need to patch it in container images
• Sync encryption keys to peers when registering a slim poller, ensuring Naemon can start correctly (it worked before due to systemd restarting Naemon down the line when the slim poller pushed its encryption keys)

[jacobbaungard]

EL8 failures seems to be unrelated to these changes, master build also fails, with the same errors.