Milter-greylist

According to Fabien Tassin:
> 
> According to manu@...:
> > 
> > > We could also change the model a little bit and test the optional features
> > > independently.
> > 
> > That seems a good solution. Are you going to work on that change?
> 
> once I have time, yes. Unless someone beat me to it :)

Just had a look. Seems easy but current code is a little bit messy.
Some tests are performed twice, and lib tests are spread everywhere.
Would you accept a patch that moves blocs around ?
(I know some reject that considering it's cosmetic).

I'd like to add a few comments in there too (1 line explaining what the
next bloc is about). Do you mind ?
 
/Fabien