Fix for bug 707, getent group for huge ads groups (>1500 members)

Fix for bug 707, getent group for huge ads groups (>1500 members)
This introduces range retrieval of ADS attributes.

I've rewritten most of Günther's patch, partly to remove code duplication and
partly to get the retrieval of members in one rush, not interrupted by the
lookups for the DN.

Andrew, you told me that you would like to see a check whether the AD sequence
number is the same before and after the retrieval to achieve atomicity. This
would be trivial to add, but I'm not sure that we want this, as this adds two
roundtrips to every membership query. We can not know before the first query
whether we get additional range values, and at that point it's too late to ask
for the USN.

Tested with a group of 4000 members along with lots of small groups.

Volker