When the `--daemon` flag is used, `lightningd` requires that `--log-file`
is used as well. By consequence, the `lightningd.service` didn't work
out-of-the-box for me.
This changes also sends the logs to `journald`.
The new approach is consistent with the `bitcoind.service` file in the bitcoin-core repository.
I prefer this approach because it comes with automatic log-rotation.