nixos: add 'localRecipients' config option for Postfix

The new option services.postfix.localRecipients allows
configuring the postfix option 'local_recipient_maps'. When
set to a list of user names (or patterns), that map
effectively replaces the lookup in the system's user
database that's used by default to determine which local
users are valid.

This option is useful to explicitly set local users that are
allowed to receive e-mail from the outside world. For local
injection i.e. via the 'sendmail' command this option has no
effect.