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.

VL rewrote 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.

I rewrote that patch, to ensure that we can keep an eye on the USN
(sequence number) of the entry - this allows us to ensure the read was
atomic.

In particular, the range retrieval is now generic, for strings.  It
could easily be made generic for any attribute type, if need be.

Andrew Bartlett
(This used to be commit 131bb928f19c7b1f582c4ad9ac42e5f3d9dfb622)