nixos/postfix: migrate options in freeform config options

This is a first step that reduces the intransparent configuration aspects
by migrating relevant options from `service`postfix` into the `config`
option that represents the `main.cf` file.

Not all options were migrated since Postfix provides reasonable defaults,
e.g. `myorigin` defaults to `$myhostname`, so it is rarely needed.

The `inet_protocols` integration with `networking.enableIPv6` was also
a thing I removed, because Postfix is capable of understand what inet
protocols are available at runtime, so I concluded it a niche option.

Every option on the `config` freeform setting now also references the
upstream documentation, which provides a great number of examples and is
always worth a look.

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