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/]
```

This was missing the fact rclone also supports listening on Unix Domain
Sockets.

It fails to build on plan9, which is part of the rclone CI matrix, and
the PR fixing it upstream doesn't seem to be getting traction.

Stub it on our side, we can still remove this once it gets merged.