More parameters + Hiera creation of parameters

This adds the oft-used parameters `mailbox_command` and `mailbox_transport`, and also adds a hash `maincfs` that can be used in Hiera to create arbitrary `postfix::config::maincfhelper` resources (say, for missing parameters).

In YAML, this could be:

```yaml
postfix::config::maincfs:
  mailbox_transport:
    value: lmtp:unix:/var/spool/postfix/private/dovecot-lmtp
```

I should note that the Puppet compilation will fail if we have already defined that parameter in `config.pp`, so perhaps we should think of removing all those that are defaulted to `undef`... and just checked, that's all of them. Hm. Thoughts? (This doesn't block this pull-request.)