Fix appending to a user's group on Solaris

Without this change, you get:
    AttributeError: 'set' object has no attribute 'extend'

Tested on a Solaris 11.2 client with the included Python 2.6.8.
Merge pull request #548 from mavit/solaris-user-groups

Fix appending to a user's group on Solaris, someone thought it was a list but it is really a set.