Added provisions for a layer between the backend and the ODBC

Added provisions for a layer between the backend and the ODBC
for further mucking with data.  This can be of use in ill situations
where not all the required massaging can be done on data with SQL
by means of stored procedures, but overlays are called too early
and cannot be used to make data non LDAP compliant.
- only support for bidirectional DN mucking is provided right now
- support for other values mucking is planned
- write is not completely tested yet
- the API could change quite often; don't rely too much on it

other cleanup has been added.