Add /etc/service entries to fix Postfix startup errors

[bagasme]

This PR will fix issue #2752.

I haven't try this guide locally, but I guessed that the error lies after configuring Postfix and before going through Dovecot configuration.

Sorry if the commit message title looks truncated and split.

[nmelehan]

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[bagasme]

Please do not merge until I fix commit message title.

[bagasme]

OK, I'd force-pushed with fixed commit message, have a CI test running.