[MERGE] Allow stacking for --dev6

[Ian Clatworthy]

Ian Clatworthy has voted abstain.
Status is now: Semi-approved
Comment:
I'm not familiar enough with this stuff to be confident of a quality 
review. Was Robert's feedback effectively a 'resubmit' vote?

If not, maybe Andrew or Martin can help get this reviewed in detail 
(with Robert likely to be offline most of the next few days).

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C4A09E5FC.7020707%40arbash-meinel.com%3E
Project: Bazaar