rmilter service: Fix a couple of bugs

  * The module uses `stringSplit` but it should be `splitString`

  * `rmilter` doesn't actually support binding to multiple sockets.
    Therefore, bind to the last one specified if `socketActivation` is
    `false`.

I also believe there is a bug in this module related to systemd
`ListenStream`.  If `socketActivation` is true, Postfix gets
connection timeouts trying to connect to one of the `ListenStream`
inet addresses.  I don't know enough about `ListenStream` passing
connections on to `fd:3` to understand what's going on.

These changes are in production (with `socketActivation = false`) via NixOps.