r10955: finally worked out why our computer accounts were being identified as users...

r10955: finally worked out why our computer accounts were being identified as users in mmc.

The problem was that the samdb module was auto-adding objectClass=user
for these accounts. That would be OK, as computer accounts are
supposed to be in that objectClass, but mmc cares about the order of
the values in the objectClass attribute! It looks for the last value,
and takes that as the value to use when deciding how to manipulate the record.

So, this patch adds an explicit objectClass=user to the record when it
gets created, which tells the samdb module to not add it as well. That
fixes the order. I suspect we are missing something else though - is
objectClass supposed to auto-sort based on the schema?
(This used to be commit 68c5f807fdb99fd605154d455e61a08293cbd2d0)