Change the winbind interface to use seperate 'domain' and 'username' feilds for

Change the winbind interface to use seperate 'domain' and 'username' feilds for
the sid->uid and uid->sid conversions.

Remove some duplicate arguments from these funcitons, and update the
request/response structures for this and the 'winbind domain name' feature.

As such 'winbindd_lookup_name' now takes both a domain and username.