Let systemd create directories

[gchamp20]

Context

Manually creating directories conflicts with systemd service hardening features. Particularly, ProtectSystem=strict mounts the filesystem has read-only for the processes started by the unit which leads to mkdir failing.

By setting User=mosquitto and adding RuntimeDirectory and LogsDirectory, systemd creates /run/mosquitto and /var/log/mosquitto with the right permissions even ProtectSystem=strict is used.

Adding User=mosquitto also has the side effect of running the daemon as the user mosquitto. I

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