attid/prefixmap fixes

[tridge at samba.org]

Hi Kamen,

I've just pushed the patches from your drsuapi-wip branch. Thanks for
all the work you and Nadya did on this!

I had to fix one problem with it. After your changes we now have
attributeID_id values of greater than 2^31, which means that it is a
negative value as a 32 bit signed integer. The attid sort which the
bisection search uses used a comparison function which just subtracted
the two attributeID values. That led to a unstable sort (ie. the sort
would end up in different orders depending on the initial state).

We need to be careful about our comparison functions, especially for
unsigned values. We've found a few bugs like this in the last few
days!

Cheers, Tridge