posixGroup membership queries from Domain

`GitHub::Ldap::Domain#is_member?` doesn't correctly validate group membership for posixGroups due to some assumptions by the `Filter#member_filter` about the DN.

This changes the `Filter#member_filter` method to accept a `Net::LDAP::Entry` object and maps the correct group type with the correlated member attr (which, for posixGroup, is the member entry's CN).

Wouldn't `uid_attr` be an attribute (string), so the map/reduce doesn't make sense? That would mean this was sufficient:

```
MEMBERSHIP_NAMES.map {|n| Net::LDAP::Filter.eq(n, entry.dn) }.reduce(:|) |
                 Net::LDAP::Filter.eq("memberUid", entry[uid_attr])
```