Instead of the listening addresses specified above, rclone will listen to all
FDs passed by the service manager, if any (and ignore any arguments passed by
`--{{ .Prefix }}addr`.
This allows rclone to be a socket-activated service. It can be configured as described in
https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html
It's possible to test this interactively through `systemd-socket-activate`,
firing of a request in a second terminal:
```
❯ systemd-socket-activate -l 8088 -l 8089 --fdname=foo:bar -- ./rclone serve webdav :local:test/
Listening on [::]:8088 as 3.
Listening on [::]:8089 as 4.
Communication attempt on fd 3.
Execing ./rclone (./rclone serve webdav :local:test/)
2024/04/24 18:14:42 NOTICE: Local file system at /home/flokli/dev/flokli/rclone/test: WebDav Server started on [sd-listen:bar-0/ sd-listen:foo-0/]
```
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