change the dsdb_control_current_partition to not include internal variables

change the dsdb_control_current_partition to not include internal variables

This structures was used in two ways. In one way it held variables
that are logically internal to the partition module, and in the other
way it was used to pass the partition DN down to other modules. This
change makes the structure contain just the dn which is being passed
down.

This change is part of the support for linked attributes. We will be
passing this control down from above the partition module to force
which partition a request acts upon. The partition module now only
adds this control if it isn't already there.