sorting out difference between aliases and groups in the cases where

sorting out difference between aliases and groups in the cases where
unix groups are not explicitly mapped.

i.e as a PDC or BDC you can have domain groups, as a member of a domain
you cannot.

as a member of a domain, unmapped unix groups are assumed to be aliases,
and as a PDC or BDC, unmapped unix groups are assumed to be unix groups.

there is _one_ other check needed with aliases to be added: unmapped unix
groups that have the same name as an NT group on the PDC (for which i will
need to write an LsaLookupNames call) should be assumed to be domain groups
on the PDC.
(This used to be commit 53b49b44e13a4ca9818ebc947372b1374831b568)