adminaddr should also set From on mails from root@localhost

In https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1062366 ,  a Debian user writes 

```
The /etc/nullmailer/adminaddr address should also define the From for messages sent BY
root, not just TO root, and use it to make nullmailer overwrite any outgoing
root@defaultdomain message.
```

It seems to be a feature request?