systemd unit file could use --verify-config in ExecStartPre

The ExecStartPre directive (systemd.service(5)) allows to run commands before actually starting the service, and to not start the service if one of these commands fail. It allows one to replicate the behavior that the tor initscript in Debian has, which is desirable IMO: if we don't have this, then when we install the systemd unit file in Debian, we have a regression.

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