Add support for trusted users and groups

```
We need a list of trusted users and groups. We could use this list to avoid 
reporting write access that the user does not care about.
On Linux root user would be trusted. Root group too probably if it had no 
members.
On AIX the user may consider the bin user to be trusted.
```

Original issue reported on code.google.com by `bernardo.damele` on 22 Oct 2012 at 9:11
